Did not see any instructions. Sometimes the system here blocks certain content....

Looks like it's just popped up now. Perhaps it was being vetted? I sent a copy by Pm through the Forum messaging service just before it got posted.

Thanks for the instructions. I will have a try later. Using MacBook Pro OS 15.1.1 and latest TB. I wonder why the other POP email in TB is not affected. The only difference I can find is that the two affected emails (POP and IMAP) are often using Mail Merge. I also had an issue earlier on with TB acting weird because of an added app on my iPad (yes!) to use two iCloud emails. Later all deleted and iPad given away. Looks like all the "weird" behavior on TB has gone, but who knows what damage it made. I will see if I can really find that trash file as those files can be difficult to open and to delete... As for the IMAP, yes also different and the location of the trash file is weird: "imap://bjprc%40mac.com@imap.mail.me.com/Trash" - that does not look like a location on the MAC... I keep you posted.

Finally found the trash files, they are "hidden" and to view them there is a special way. See the screenshot with the two trash files. As the trash is empty, yes 0 k. See the other POP account (strategy4china) that has no problem. Did not delete anything yet. Comment? So, delete the two files? Somewhere I also saw stuff related to IMPAM but did not check well yet.

I *never* said to delete the files. I am just a little concerned that you haven't fully grasped what I'm asking you to do. I said to save these files into a folder on your desktop in case they were required. Please make sure you follow the instructions I gave - or don't complete the process at all. My instruction shows how to regenerate the Trash Folder just in case your problem was that it was corrupted and not allowing compacting.

I did not explain correctly. By "deleting" I meant to follow your instructions. I wanted first to show you how the files look like

The files look like they do on mine. Just save them and don't delete them. In other posts, some helpers are noting that they think the error message is caused by anti-virus software. I guess that you have already made an 'exception' for the profile folders but that would be another thing to try if this doesn't work. Another - which would be a diagnostic only - would be to start the computer in Safe Mode with Networking and see if the problem persists. If TB is okay, that only tells you that it isn't Thunderbird - the solution can be down a very long road! But, let's see if the Trash Folder recreation works first. Finally, this is a problem that a fair amount of people have been experiencing.