500 Servlet Exception

When I delete unwanted emails, move emails to different folders, or sometimes after reading a email; I get locked out with this exception.  I emailed Cox and they suggested it was my files on the computer or a Java problem.  I used my computer at work and the same problem occurred.  So that tells me its not my computer.  My home computer is a new windows 8 and for the first month no problem occurred.  My old computer did not have this problem last year.  My work computer is a older windows 7 professional.  What do I do?  Whom do I call?  After 24 hours the problem clears and I can get email again.  This is annoying!  I would appreciate any help or advice.

Thanks,

Marshall T