I did notice in the Troubleshooting Information these entries:

         * Lightning	68.7.0	false	{e2fda1a4-762b-4020-b5ad-a41df1933103}
         * browser.cache.disk.amount_written	1930147
         * browser.cache.disk.capacity	1048576

Could this be the issue, that 'amount_written' has exceeded 'capacity?

You should have Lightning 68.7, and the Troubleshooting entry should be true. I suggest you follow the reset instructions here, in case you didn't perform that exact step before. The Config. editor is under Tools(or AppMenu/Options)/Options/Advanced/General. After the reset, check that Lightning is v. 68.7.

I don't think anything in the .../Local/Thunderbird folder is relevant, as the problem probably lies in the profile folder, which is in .../Roaming/Thunderbird.

You should have Lightning 68.7, and the Troubleshooting entry should be true. I suggest you follow the reset instructions here, in case you didn't perform that exact step before. The Config. editor is under Tools(or AppMenu/Options)/Options/Advanced/General. After the reset, check that Lightning is v. 68.7. I don't think anything in the .../Local/Thunderbird folder is relevant, as the problem probably lies in the profile folder, which is in .../Roaming/Thunderbird.

Thanks for responding. After my post I checked the .ics file by opening it in Notepad. The only special characters I found were the $ sign, hyphen, comma, and period. I did notice that if the dollar sign was followed by a single digit and comma, e.g., $2,000, that the comma was escaped thusly: $2\,000 otherwise it was not, i.e., $2000 was untouched. Next I closed the .ics file and double-clicked it. Outlook loaded the calendar file immediately. Apparently Outlook was set as the default for .ics files -- I didn't realize that. Anyway, it opened without a hitch so I guess the .ics file may not be the issue, although I did notice a couple of events with lengthy (160 entries) series of EXDATE(s).

Here are my results from your above link:

   DONE Go to the Advanced section of Thunderbird Options and click on Config Editor...
   DONE Copy or start typing extensions.installedDistroAddon.{e2fda1a4-762b-4020-b5ad-a41df1933103} into the search box
   DONE When extensions.installedDistroAddon... is listed in the preferences list, right-click on it and choose Reset from the context menu - the preference now isn't bold anymore and its value is empty
   DONE Close the editor, leave the Thunderbird options, open Thunderbird's add-ons manager and switch to the list of installed extensions
   DONE-REMOVED If Lightning is listed there, click on Remove to the right of the list item
   DONE Restart Thunderbird
   FAIL-HANG After Thunderbird has been relaunched, Lightning will be back and you will have access to your calendar data.
    

On restart Thunderbird hangs. The app is non-responsive, the tile bar displays "(Not Responding)". The Lightning icons are visible, but not accessible. At this point previously I could exit Thunderbird but only restart it using Safe Mode. A normal launch would just hang.

On a whim I tried minimizing Thunderbird and discovered two of the three, control buttons at the upper right corner of the Title Bar function: Exit and minimize. After minimizing, I clicked the Task Bar icon to bring Thunderbird bringing it back into focus and clicked the X to exit.

Then I relaunched Thunderbird and to my surprise it launched normally, but without the Lightning icons. Tools > Add-ons showed Lightning loaded. Thinking there may have been an update issue, I closed & restarted Thunderbird. HANG.

This time there were no Lightning icons showing. The only controls available were minimize and exit. Minimize > bring back window > Exit. Launch... looked OK for a second, the Lightning icons were visible, then it hung again with two controls available, min/X. Minimize > focus > Exit > launch ... got about 3-seconds of "normal". There seems to be some sort of process going on in the background. I do recall earlier seeing Lightning and Thunderbird were not at the same revision level, Thunderbird is on 68.7 but Lightning was behind that. I am wondering if the issue is Lightning trying to update in the background and just taking an oddly long time to do so? Thinking this could be a possibility I left Thunderbird minimized in its hung state for about 20-min. Task Bar Thunderbird icon > (window in view but still hung) > exit. Re-launch, same behavior, same 2-3 seconds to hang.

Re-launched in Safe Mode, IMAP new messages populated. Clicking Lightning in Add-ons showed the current version is 68.7.0 last updated April 27, 2020. [Disable] > exit Add-ons Manager > Exit Thunderbird > Relaunch and everything works except of course, Lightning is disabled.

Set all email accounts to not check server, restarted to confirm no new messages are loading, enabled Lightning, closed Add-ons Manager and restarted Thunderbird (File > Exit). Between 4-5 seconds after launch, Thunderbird hung again. No lightning icons were visible and the behavior of controls was the same. Safe Mode > disable Lightning > restart, Thunderbird is working again but no Lightning.

I don't know if any of this helps or it was just an adventure down a rabbit hole.

Sky

Are any of your calendars network calendars such as provided by Google, that usually require the Provider add-on? Does this happen with any other holiday calendar besides the one from WinDates?

Are any of your calendars network calendars such as provided by Google, that usually require the Provider add-on? Does this happen with any other holiday calendar besides the one from WinDates?

I only have the one calendar and it is local. I don't have any holiday calendars. I chose that method because it looked like a generic, "This is how to import any (______) calendar in .ics format." sort of instruction.

…the problem probably lies in the profile folder, which is in .../Roaming/Thunderbird.

With the last auto-update of Thunderbird, Lightning re-enabled by default and all of the problem behaviors appeared again. My work-around still works, so I safe-started Thunderbird and disabled Lightning and went through the whole removal sequence yet again.

I have tried everything suggested, everything I can find in the forum, and so far nothing has helped. I am stuck with my work-around and no calendar. I looked in the profile folder(s) but could find nothing that jumped put at me and plainly stated, "I am Lightning, or I am the .ics file you tried to import.". I am thoroughly stuck.

1. Does it make a difference that I have one POP3 account which is not set to automatically fetch mail and three IMAP accounts?
2. Is there a calendar alternative for Thunderbird that is more reliable?
3. Is there a way to actually remove all files Lightning is referencing and clean it out, then re-install it without losing all of my email?

Besides the calendar data in the calendar-data subfolder of the profile folder, the Lightning add-on is the file {e2fda1a4-762b-4020-b5ad-a41df1933103}.xpi in the extensions subfolder. I would suggest you create a new profile from Help/Troubleshooting, about:profiles, add one account, and import your .ics calendar. If that works properly, it indicates there's some unknown issue in the original profile - which can be opened as well from about:profiles.

I'm not aware of an alternative to Lightning for TB 68, although there are add-ons that provide limited functions such as mailmindr.

Besides the calendar data in the calendar-data subfolder of the profile folder, the Lightning add-on is the file {e2fda1a4-762b-4020-b5ad-a41df1933103}.xpi in the extensions subfolder. I would suggest you create a new profile from Help/Troubleshooting, about:profiles, add one account, and import your .ics calendar. If that works properly, it indicates there's some unknown issue in the original profile - which can be opened as well from about:profiles. I'm not aware of an alternative to Lightning for TB 68, although there are add-ons that provide limited functions such as mailmindr.

EXECUTIVE BRIEF I tried importing the .ics calendar as suggested (I think) and it failed

LONG VERSION Followed above instructions, created a new profile called "TEST". After setting it up I ignored the email setup and went straight to importing my events.ics file. That resulted in:

An error occurred when writing to the calendar Home! Please see below for more information. MODIFICATION_FAILED If you're seeing this message after snoozing or dismissing a reminder and this is for a calendar you do not want to add or edit events for, you can mark this calendar as read-only to avoid such experience in future. To do so, get to the calendar properties by right-clicking on this calendar in the list in the calendar or task view. [ OK ]

Then when I tried to close Thunderbird I got this error:

3 item(s) were ignored since they exist in both the destination calendar and C:\Users\username\Desktop\events.ics. [ OK ] and on clicking OK, the error #1 noted above appeared again. Clicking OK this time allowed Thunderbird to close.

After this, reopening Thunderbird always opened TEST; it complained about not having an email setup; it refused to display any way to try importing the events.ics file again; etc., etc. It refused to let me delete the profile from troubleshooting > about:profiles. I went to the folder, closed Thunderbird and tried to do it there (failed); modified the .ini to Lock="0" and tried (failed); went back into Thunderbird > Troubleshooting > about:profiles noticed the "set as default" buttons (durp), changed the default profile to the latest used per the File Explorer, restarted (nobody home - useless profile left when setting up Thunderbird); back to help... set Default to the correct (not TEST) profile, restarted and that worked. Back to help... deleted TEST profile and stopped there. Probably should have cleaned up the other profile as well but didn't.

Sorry this took me awhile to get to. I'd gotten my system working 'sort of OK' and had stuff to do until a couple of days ago when Thunderbird once again enabled Lightning in the background. It took me awhile to recall my fix and this thread.

Besides the calendar data in the calendar-data subfolder of the profile folder, the Lightning add-on is the file {e2fda1a4-762b-4020-b5ad-a41df1933103}.xpi in the extensions subfolder…

I found {e2fda1a4-762b-4020-b5ad-a41df1933103}.xpi in Roaming\Thunderbird\Profiles\nwipuw2i.default-release\extensions\staged. This was in two of my email accounts: an IMAP and my POP3. I also found tbsync@jobisoft.de in the same folders (…\staged\).

1. Could the root of the problem come from selecting the wrong email account with which to associate the calendar?
2. Do I have to associate Lightning with an email account?
3. Can one Lightning calendar be associated with more than one email account, e.g., 1-to-many?
4. Is Lightning setup to be in a 1:1 relationship with an email account, thus giving a user with three email accounts the potential of three different calendars?

Sky

Lightning is installed for all accounts in a profile, but each calendar has an associated mail account. Right-click a calendar in the left pane of the Calendar tab, Properties.