Due to a recent security fix, some problems may be introduced with existing 1.5 sites. The know issues are as follows:
Invalid Token page displays when trying to login
Older 1.5 templates, 1.0 templates working in legacy mode, or those using customer login code or using layout overrides will experience this problem.
I'm getting this error with Firefox 2 on the Backend. The Backend Template is the original Joomla 1.5 Template. This can't be the reason for this error.
But whats then the reason?
NOTE: Message Icon changed to Solved