If you see this message, along with the error code ERR800 it means there was a problem accessing or creating your inbox. While this can be usually resolved by starting over, there are a couple of things you can try if that doesn't immediately work.
Restart your browser:
Sometimes the old "turn it off and on again" solution actually works! Just close out whatever browser you use, then start it back up again. Once the browser has been restarted, go to to mail.aol.com and enter your username and password.
Clear your cache, cookies, and history:
Another option to fix the error messages would be to clear your browser's cache, cookies, and history. Doing this will wipe out all the little unwanted bits of info that have been stored in your browser, resetting it and making it overall run better.