The solutions mentioned above didn't work out for me.
I was able to solve it by deleting the password of the problematic imap email account stored in the Thunderbird Password Manager. My guess is that some files used by the password manager are corrupted.
Before you try to disable SSL etc. in Eset Security try to reset the password of the email account first, it is stored in the Password Manager of Thunderbird under Options > Privacy and Security > Saved Passwords.
Delete the password for the problematic imap email account. Overwriting the username and / or password will not work.
Then
check for new messages on the server and enter the password when prompted (*).
send a test email to the problematic imap email account (send a message to yourself) (*)
(*) Optional: enable the checkbox to store the password in the Password Manager
