Could it be because of wayland?

No, I have tried on X11, I face the same problem.

Rebuilding the Global Database didn't help...https://support.mozilla.org/en-US/kb/rebuilding-global-database

Any other idea?

Could it come from lightning?

Hello,

I believe I have found something related to this problem in my Thunderbird error console :

Lightning: Couldn't find /freeassociation.sourceforge.net/Europe/Paris

[calExtract] Faulty extraction pattern from.hour.minutes, missing parameter in calExtract.jsm:1240

Does it help?

Thank you,

The error seems to refer to the Free Association calendar app, which is now libical. I don't know exactly how this relates to Lightning.

I had found these two names without being able to make a relation. Thank you for this information. Do you have any idea how I can debug. As a reminder, my thunderbird profile is rather old (I'm using Thunderbird for about 15 years). It has evolved along Ubuntu till recently when I decided to migrate from Ubuntu 18.04 to Debian 10. Could it be that Ubuntu 18.04 was using Free Association and Debian 10 is using libical and my profile folder didn't "understand" this change?

It's easy enough to create a new profile, add an account and calendar, and see if the error remains. If it doesn't, it's not too much trouble to transfer data from the old profile to the new one, and you might see other benefits. Creating a new profile for each major release of TB is probably a good idea.

Help/Troubleshooting, about:profiles, to create and manage profiles.

Thank you!!!!!!!!!!!!!!!!!!!!!!!!!! I created a new profile with an email and a calendar (i tried both local and caldav) and the error wasn't there. Converting an email to event worked smoothly.

So now that the problem is identified, how can I solve it. How to transfer data from my old profile to a new one? Email addresses and extensions are OK, I think I can do most of it. My biggest worry is about my local folders? How to migrate them? I'm going to look for tutorial, I'm sure the are. Thank you @sfhowes.

To transfer local mail, open the old profile folder, then open the Mail (POP accounts and Local Folders) or ImapMail (IMAP accounts) subfolders, and then the subfolders named after the mail server, e.g. pop.gmail.com or imap.mail.yahoo.com, and Mail/Local Folders, as well as .sbd folders. Look for the mbox files - large files with no extension named after a folder, e.g. Inbox, Archive, Sent etc. Copy the mbox files, renamed if necessary, into Mail/Local Folders of the new profile, while TB is closed. When TB is restarted, the folders should appear under Local Folders in the Folder Pane.

http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Thunderbird

Thanks. What about ._NAME binary files? Should I copy them too? They seem very old (2012). And .msf files?

I don't know what those binary files are. An mbox file is accompanied by a like-named index file with an .msf extension, but they can be ignored when copying to the new profile (they are automatically regenerated when TB starts).

Migration is done and everything seem to work for the best. Thank you so much!