Internal error 500 after enabling plugin (error in first post)

hi all , after enabling plugin I get this error : (I can't even login to back-end)


Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/[user/]domains/[domain.com]/public_html/index.php:2) in /home/[user]/domains/[domain.com]/public_html/libraries/joomla/session/session.php on line 492




Fatal error: Call to a member function isAdmin() on a non-object in /home/[user]/domains/[domain.com]/public_html/plugins/system/mobile/mobile.php on line 373


Any help would really appreciated . 

What Joomla and MobileJoomla versions do you use?

And what is plugins/system/mobile/mobile.php plugin? It's not part of MobileJoomla.

Thank you for your answer. I'm using joomla 2.5 and joola mobile 0.92 . I disabled the plugin  in phpmyadmin and therefore i managed to get access to back end. after that i uninstalled the plug in and that solved my problem. i will test the newest version now.