Apple Mail Error "message --- could not be moved to mailbox "AOL"

I want this fixed and am willing to try anything. I've already deleted and added the account back. However, take into consideration It took 3 days to reload my email account mailboxes. I have to keep my MPB from sleeping or logging out the entire time for the downloads to complete uninterrupted. I'm hesitant to use my computer, wifi, applications, etc., the entire time so I don't interrupt reloading the AOL account. The path to server settings in Mail (preferences>accounts>server settings) yields one option "Outgoing mail account: AOL" and nothing else so the Mail Settings from Apple Support are not useful. I have a separate user account with none of the settings, preferences, etc., from my main user account - problems exists there too. So, are there no other solutions? Is anyone else having this problem and found one? Thanks again, Rod

Update . . . since I booted in Safe Mode and then returned to normal boot, the Mail error has not reoccured. It's only been 4 days but that's long enough to suggest the boot/re-boot fixed it. So as of today, I'm a happy camper! I'll let you know if it changes. Thanks for helping!

Thanks Michelle. I have not booted in safe mode but will. I have a second user account but it has none of my personal information, documents, Quicken, preferences, etc., so using it long enough to discover the mail problem is not practical. The occurrence of the error messages is unpredictable. I cannot determine what causes them or exactly under what conditions they occur, but it happens every 2-3 days to the point I have to force quit Mail and reboot my MBP to use Mail. I do not any other mail accounts. I've already exhausted all the Apple Support articles and Google searches . . . the fixes are too old or don't apply to Big Sur. My MBP is too old (Late 2013) to upgrade to Monterey. I'll do a safe mode boot. It may take a few days to see if the problem persists. Hmmm . . . Rod

Thanks for responding. I booted in Safe Mode yesterday evening and have done typical mail activity. The Mail error messages have not reappeared but as I noted, that could take a few days. Instead, I am experiencing a number of issues in Safe Mode: Chrome is missing navigation tools and not opening new windows, got logged out of my user account, slow app responses/scrolling/windows opening, screen freezes after using the red/yellow/green navigation buttons in app windows - have to force quit the app. I noticed this morning when I opened my MBP, it was very warm to the touch - I left it in Safe Mode charging overnight as I often do. Not sure what this is all about but I’m not sure I can tolerate these issues in Safe Mode long enough to see if the Mail errors reappear. Hmmm . . . any suggestions or any insight about what's going on in Safe Mode?

Thanks Specialists! I could not tolerate the issues I described in Safe Mode so restarted. None have reoccured. I did not see the Mail error message while in Safe Mode and so far, they have not reappeared after I re-booted. I don't know what any of this means. I will continue to use Mail as I normally do and test if the error returns within the next few days. I don't understand the temperature rise either. I have not changed anything but reviewed the support page you sent. The temp seems normal now that I'm out of Safe Mode - strange indeed. I'm on a fixed retirement income and can't afford to pay Apple for support so I came here. Perhaps someone else having this problem will see it and respond. Am I absolutely the only person having this problem?Should I re-post it in a different Support Community? Again, thanks for trying to help.

Community Specialists: Unfortunately, the fix didn't stick and the error message (#2 above) returned twice. I was working on a draft email over a period of several sessions. I started the email, saved it a a draft, worked on it later and then attempted to finish it the next day. So far, the error occurs only while drafting and involving the "APPEND invalid credentials" error message. Darn!

Good to know I'm not alone but sorry others have to endure this as well. Unfortunately, my error messages seem to be increasing. I'll try a re-boot in safe mood again and see if that works at least temporarily. So far, that seems to be the only relief. Contacting AOL is frustrating and almost every time I have in the past, they point the finger back to Apple. So . . . if any of the community specialists or experienced users in these rooms have ideas, I'm game!