Thunderbird is stuck on "loading messages", but only on one of my two identities,

Thunderbird 44 is so old I an I guess everyone else in this forum barely remember it.

Beyond that you use of the term "identities" does not appear to be align with the usage in Thunderbird, where an identity is just something within the account. Largely used in replying to mail where multiple accounts/addresses have been aggregated into a single one.

So I am relying on you to clarify if I do not understand. I assume you are referring to two separate accounts in the Thunderbird folder list and you are swapping between the inbox folder of them.

The problem account is it imap or POP? My guess is IMAP but it is only a guess based on the error you report.

Who is the mail provider for that account? It sounds like there may be a connectivity issue between you and the server, but what error does not have an error message, that is the real question.

SO I am back to the old standbys.

Do you have an antivirus? Does it scan email? What happens if you disable scanning?

What is the mail provider? Have they changed the security requirement for connecting to their mail server and Windows XP and Thunderbird 44 are not incompatible with their requirements.

Yahoo and Outlook/Hotmail have recently changed authentication requirement to those that Thunderbird 44 would not support.

My understanding is XP lacks some of the more modern encryption protocols that are now common. For instance XP only supports TLS 1. That was deprecated in Thunderbird years ago and most web servers and mail servers now refuse to use any encrypted connection method that XP supports.

This topic from 10 years ago offers some diagnostic information that might help. https://support.mozilla.org/en-US/questions/1132925

As for locating your profile, that has not really changed much since V3. So use the troubleshooting information in the help menu to locate it. Click the open button against the profile entry.

I'll answer your questions and then I'll mention another clue I just came across that might dovetail with your suspicion.

By identities, I mean accounts. I have 4 email accounts with the same provider, Spectrum/Charter...and two of them are set up in the version of Thunderbird that I'm using. I have the same Thunderbird on another computer and have two accounts on it. They all work fine except for the one (jbclem4) with the problem. I have update turned off and I'll stick with this version as long as it works. I also have my main email account(jbclem1) on this same computer but using Outlook Express.

I use IMAP with some of the accounts, and POP3 with others. This problem account was IMAP, but I changed it to POP3 the other day to see if the problem would go away, but it didn't.

Antivirus...I use antiviruses manually, so there's nothing scanning my email.

The connection security has been set at SSL/TLS, and I've also tried with None for security, but same problem. Other email accounts are set at SSL/TLS and working fine with Spectrum/Charter. The outgoing server is set to STARTTLS.

Here's the new clue I mentioned...I went to create a new email account, to see if the problem would vanish, but the attempt produced a page with this error message which keep repeating itself as long as the page is open.

"Sorry - we're unable to communicate with our sign-up server"

I looked at the diagnostic information you linked to...I haven't tried starting the WinXP in safe mode/with networking. Should I try that next or is there something I should do first?