Squirrelmail webmail sudden login problem !?

[prisfeo]

ispconfig3 + Cento5.4, all works.

squirrelmail has worked perfectly till now.
now suddenly, when i access (with every configured accounts)
through the usual: http://myserver.example.com/webmail
i keep getting this error:
"Your session has expired, but will be resumed after logging in again."
and i cannot login..

i have cleaned browser cache, restart browser..always same problem..
i have not touch any code/files related to apache/squirrelmail
apache is on as normal.
(prior to reboot the server i have to understand if it can be a runtime problem)

what can be the problem ?

thanks in advance.