Do you have an antivirus scanning encrypted connections or email specifically. Both actions can cause issues as you describe if it goes wrong and it appear to go wrong a lot if the reporting in these forums are anything to go by.

I have a virus scanner and a VPN connection, but I deleted both from the machines. I also restarted the router with factory settings. This was my first thought too, but unfortunately it did not solve the problem. The strange thing is that this error appears on three different machines, even with a fixed IP address. However, Roundcoube webmail and Outlook iOs work flawlessly, but Thunderbird does not.

I just realized that all of those events happened at exactly the same time. 20 hundred hours.

My hosting provider has discovered the error, but they can't fix it because it's not a problem with the emails. The error is caused by the fact that the emails from previous hosting providers were compressed in some way, and the current Thunderbird version is unable to decompress and display them. Since Roundcube Webmail, Bluemail and Outlook all connect to the IMAP account without any problems, and all emails are visible, the error must be in the Thunderbird client. It may just be a small setting that needs to be changed, but I don't know where to look for it. I haven't had this problem for a few months, but it started after the updates.