HELP!!! Header already sent...

[eluser]herlos[/eluser]
Hello,
I'm just freaking out!! Never had this error before and can't find the reason...
Since 30 minutes, I get the following error, when I open a page:

A PHP Error was encountered

Severity: Warning

Message: Cannot modify header information - headers already sent by (output started at E:\xampp\htdocs\eventsmanager\application\controllers\admin.php:1)

Filename: libraries/Session.php

Line Number: 671

Could you please help me?? Already removed ALL the code from my controller, excepting the constructor, but still get that error. Like I said, never got that before...

Thank you!

Johannes