I've just tested enabling addition column headers in Tasks and restarting Thunderbird. All worked ok.

Do you have a product like CCleaner or similar that is allowed to delete/cleanup session files ?

If yes, then it is advised you do not allow such products access to any Thunderbird file or folder.

Try deleting the current session.json file to see if a new one works better.

In Thunderbird

• Help > Troubleshooting Information
• click on 'Open Folder' button

A new window opens showing contents of profile name folder.

• Exit Thunderbird now.

Delete these files:

• session.json
• xulstore.json

Start Thunderbird.

Select additional columns in Tasks.

Exit Thunderbird

allow a short time for background processes and then restart Thunderbird.

Please Report back on results.

Thanks. I tried again, and worked fine.

Looks like that day something weird was happening with my computer/Thunderbird.