https://bugzilla.mozilla.org/show_bug.cgi?id=1592407#c22

Tools/Options(Mac:Preferences; Linux:Edit/Preferences)/Advanced/General/Config. editor, double-click general.useragent.compatMode.firefox to true.

This should also fix google calendar and contact sync. in 68.2.1 until updated Provider and gcontactsync add-ons are released.

This didn't fix my issue. If I go to manage the addons, I see that Provider for Google Calendar has been disabled because "Provider for Google Calendar is incompatible with Thunderbird 68.2.1".

I assume this will be fixed soon as a lot of people rely on this functionality.

tc68 said

This didn't fix my issue. If I go to manage the addons, I see that Provider for Google Calendar has been disabled because "Provider for Google Calendar is incompatible with Thunderbird 68.2.1". I assume this will be fixed soon as a lot of people rely on this functionality.

Install the latest Provider add-on, v. 68.2.1:

https://addons.thunderbird.net/en-US/thunderbird/addon/provider-for-google-calendar/

Thanks! It had updated in the background but I needed to restart Thunderbird.

Now back to full function.

Here is what worked for me:

Go to Tools->Options menu, select "Advanced" icon, select "General" tab, click "Config Editor" button,click "I accept the risk!", search for "general.useragent.compatMode.firefox" if it shows "false" double click to change to "true", enter your credentials one LAST time when it prompts. The repeating issue stopped for me. No idea why this works??

I would like to thank Yew Kay Yan for this process! It was driving me crazy having to enter Google Credentials Over and Over and Over...