Hi febert. Have you looked at your settings in Thunderbird for the E-mail account? It sounds as if your "Archive" settings are moving messages older than ?? to the Archive folder - which is Thunderbird doing its job. You can access the settings through "Tools > Account Settings > Copies & Folders". Look there and see if TBird is set to archive message and where.

I looked there and do not see that as an option. I use 52.8.0 (32-bit) of T-bird. It only gives option of archiving to "single folder, yearly forlder or monthly folder." I see no cut off date option.

Message in Archives "should" only be moved manually, as outlined here -https://support.mozilla.org/en-US/kb/archived-messages So even if you are using the settings, TBird will not move them automatically.

Are you using IMAP or POP3? Take a look at this thread - https://support.mozilla.org/en-US/questions/1123973

IMAP handles mail differently. If you are used to downloading your mail to your machine and keeping it indefinitely, I suggest you read that referenced thread and follow the instructions.

Personally, I only use POP3 and have for more than 20 years. IMAP has specific qualities over POP3, but nothing that "I" find of value. I can access mail that I've saved since the late 90's because it's on my machine and never deleted.

I understand that you have not made any changes. But your server (AOL) "may" have made changes to their E-mail server. Some of the changes AOL makes can effect your accounts - especially IMAP.

Taking a closer look at the type of E-mail account and your settings will be your only recourse. Even if you didn't make any changes, you may need to make them now.