I had the same problem, Thunderbird version 78.5.0 on a Windows 10 computer no longer receiving emails from an IMAP account and getting an error message when sending (message read: "Your message has been sent, but due to network problems or file access errors, a copy has not been placed in your sent messages (Sent) folder.").
As suggested above, I disabled SSL in ESET, opened Thunderbird and got my emails from the IMAP server. Closed Thunderbird, re-enabled SSL, reopened Thunderbird and so far it still appears to be working.
