Try to rebuild the index file of the troubled folder.

Right-click the folder - Properties - Repair Folder

Hello!

it kind of helps for individual situation, but issue is back again next day with another messages...

What is your account type - POP or IMAP?

Are you running any kind of anti-virus software?

It is an IMAP account. I do not use any antivirus.

A possible cause of the problem you're seeing is that Thunderbird 128 is shipping with a broken backend, causing IMAP folder corruption under some circumstances.

Currently the only remedy is repairing the affected IMAP folder (Right-click, Properties, Repair Folder), which usually fixes the issue.

Work is underway to understand the problem, but at this time no fix is available yet. For more details see https://bugzilla.mozilla.org/show_bug.cgi?id=1890230

Hello! it [repair] kind of helps for individual situation, but issue is back again next day with another messages...

https://bugzilla.mozilla.org/show_bug.cgi?id=1890230#c105 has a potential fix, if you would like to try it and offer feedback.

Hello Wayne,

what is the Thunderbird Daily? Is it some beta version?

Hello Wayne, what is the Thunderbird Daily? Is it some beta version?

Daily is pre-beta. But you do not need to use daily. The "try build" executables at https://bugzilla.mozilla.org/show_bug.cgi?id=1890230#c105 are version 128.

I do not think that my tech-knowledge is well enough to do it on my own. I will wait till the update with the fix in it. Till then I will just repair the problematic folders.

Fixes are now available in 128.3.3. First right+click on broken folder > properties > Repair. Then update to 128.3.3 with Help > About. Please post your results here.

marcin, are you still using version 128?

I am on 128.3.3esr (64 bits)