I suggest logging on to online account, as a message may be problematic that needs to be deleted.

Didn't find anything that looked suspicious. The behavior seems odd: one minute it says downloading 1 of 709 and gets stuck and quits. Then a minute later when I try again, it says no messages to download.

I suggest you disable your antivirus mail scanner and try again. Generally this is an issue only with POP mail accounts. If you are not using POP. Please say so.

There are all sorts of "workarounds" to the fact antivirus products encounter a malformed email and drop dead in their tracks. Sometime a Thunderbird restart will allow them to recover, other time a system reboot is required. But they will continue to drop the ball on every encounter with the mail that has caused the issue in the first place.

There is what David said already, there are suggestions to empty online spam folders (they are most likely to be malformed) but in reality the only sure fire way is to download your mail without the failing antivirus component messing it up. Once the download occurs, you might not have an issue after you re-enable it for years.

Personally I use windows defender and no mail scanner. So I have no issues with antivirus dropping the ball. But many find comfort in having their mail scanned and have to deal with the issues doing so can cause.