Just a guess, but it sounds like your settings are not correct somehow. Do you get and error dialog in Thunderbird?

No there is no error dialogue - just the entry in the error console. I have not changed the settings as far as I know. There have just been some automatic updates that I didn't want. I tried to figure out a way to disable that but it doesn't seem to have worked.

I just ask it to check the messages and it says "(e-mail address) Host contacted, sending login information..." and sits there waiting forever. It seems like a bug to me, but I can't remember if I might have done anything else to cause the issue to appear.

There are a few other errors in error console, like this one: "13:27:26.639 Error: Cannot find loader for bootstrap XPIProvider.jsm:1920:15" and: "Uncaught (in promise) Error: Error while loading 'jar:file:///home/brian/.thunderbird/3c0gfhck.default/extensions/%7BF8147CF4-B9E3-445B-AA87-081ED66548F8%7D.xpi!/manifest.json' (NS_ERROR_FILE_NOT_FOUND)

   readJSON resource://gre/modules/Extension.jsm:737
   onStopRequest resource://gre/modules/NetUtil.jsm:128"

I have tried disabling all the extensions and re-starting Thunderbird and I still get the errors.

Bump.

This bug sounds somewhat similar, but I may be wrong. https://bugzilla.mozilla.org/show_bug.cgi?id=1778689

It does not seem to be a general problem though. You can try to create a new profile using profile manager. https://support.mozilla.org/kb/using-multiple-profiles

Then use profile manager and start Thunderbird with the new profile. Re-create your account in the new profile.

Is there any difference?

Note, creating duplicate topics for the same problem is unfriendly behavior and will often just waste the time of people trying to help.

It doesn't appear to be that error. The person who reported that said it was fixed after further updates. My issue is still there. That one has been closed and marked "RESOLVED WORKSFORME". It doesn't work for me. I don't want to have to set up a new profile because I need to be able to see my older/existing e-mails.

I don't want to have to set up a new profile because I need to be able to see my older/existing e-mails.

Standard diagnostics. Either do it or not, but progress will not be made if you decide you don't want to perform the suggested steps.

I don't believe that would be standard diagnostics. That would be making a change. It is also a change that should not be needed. I have been using this profile for years with e-mails going back a long way, that I do not want to lose. It is a valid thing for me to say that I do not want to lose e-mails. I have e-mails even going back to 2006! Diagnostics would be what I provided where there is a very specific entry in the error console. I have done some software development work myself so I know that in theory it should be possible for a software developer to look into where the error message could have come from and specifically when attempting to download new e-mail messages using POP3. That is quite a specific action that is being performed.

Not sure what just happened but e-mails have just started downloading again.

Funny how you did nothing nor did Thunderbird and it is fixed.

I guess the problem was another source, like third party software (Antivirus, VPN firewall, phishing protection etc) or your provider was having issues. Did you ever ask them?

BTW, When the pipe is blocked there is no point pulling the tap apart to make water run. But how do you get to know that unless you take the tap of the pipe. Diagnostics is way more than just reporting error numbers.