[Typo3] Cannot login to BE: "Warning: Cannot modify header information"

[Diederik van Veen]

Hey guys,

just checking an older website (Typo 3.5) and noticed i cannot login to the 
BE. The eror messages displayed after login are like:

Warning: Cannot modify header information - headers already sent by (output 
started at /home/users/o/o/myaccount/www/typo3_src-3.5.0/typo3/index.php:4) 
in 
/home/users/o/o/myaccount/www/typo3_src-3.5.0/t3lib/class.t3lib_userauth.php 
on line 150

Any ideas what could be the reason?

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