We can't tell you what the ticket means ... we have no way to see the ticket without you showing it to us.
If you have console access to the system, the ticket is a text file (HTML, actually) in [web2pydir]/applications/myapp/errors or possiblyin [web2pydir]/applications/admin/errors. If you don't have console access, things will be rather tough.
First guess is that there is something in the session folder that's an older format session, and it can't be unpickled. There was also a format change on tickets several releases ago, but that's less likely to be the problem here.
Good luck!
/dps