Your screen shot is super hard to read. Can you post an improved version? And what is the text of the warning in English?

Wayne, thank you for answer. Here is the warining: Encountered a corrupt folder imap://<mailbox>@<server>/INBOX/Drafts

where <mailbox> is the email address of certain mailbox.

And now I cannot send anything now - sending a message hangs up on copying it to the Sent folder

Another try to solve the problem - I deleted my TB email account configuration and added it anew. The same problem appears....

I have had something of a shocker upgrade as well. SO I think from what I have learned we should start by seeing if the actual physical file that contains the "corruption" actually exists.

Open the troubleshoot information on the help menu. Click on the show profile button in the first block of information Shut down Thunderbird once the file manager/explorer opens.

Open the IMAPMail folder and the subsequent one with the server name mentioned in the error message after the @

There should be an inbox.sdb folder. Open that.

What files do you see named drafts? (I expect an drafts file and a drafts.msf file) If what you see matched what I expect copy both files into a USB drive (or your desktop) and once done with the copy delete them Restart Thunderbird.

I asked you to make the copies for a few reasons. One is that if this goes pear shaped you can always put them back. The second is if there is important stuff saved as drafts that is not recreated when you restart Thunderbird, it may be possible to extract it from the corrupt file. Finally it may be (unlikely) that it might have further use in diagnosing the issue by a developer. But as I normally don't go there unless I see a trend (you are my first) it is unlikely you will be asked.

Please report your results. I want to know if I am guessing even close and if things are working for you. Post upgrade issues are the worst. From everyone's perspective I think.

Who is your mail provider?

If also hangs on Send user needs to make sure a valid Sent folder is set in "Copies and Folders" settings. Also, make sure the Drafts folder is set correctly on the "Copies and Folders" screen under "Drafts and Templates". (The bug report at https://bugzilla.mozilla.org/show_bug.cgi?id=1776613 indicates the problem only occurs for Drafts.)

mirek1, Who is your mail provider? Is this only with the drafts folder?

The solution provided by Mat works :) It was quite a lot of work where is almost 30 mailboxes however works.

Answering to Wayne Mery - I've got 2 providers - kei.pl and smarthost.eu. And it happens only with drafts folder.

I was too optimistic... Unfortunately after several opening and quiting TB the problem with this drafts files came back :(

I was not offering a solution for 30 accounts, I was trying to determine what the problem was. After you restarted Thunderbird all of your drafts folders should have been recreated.