Mantis is one of the most commonly used open source bug tracker in the software industry.
Sometimes users might complaint that they are getting following type of error message while logging off:
APPLICATION WARNING XXXXX: Your session has become invalidated.
Why the error occurs?
If you are behind a web proxy that uses multiple exit points, then session validation will fail, as your session looks like it is being hijacked.
Uncheck the "secure session" option when you log into Mantis to turn off the validation. Alternatively, you can set $g_session_validation = OFF to disable validation globally.