[Typo3] MySQL Error

[Marc Valentin]

I get the following error message when I try to access my website, 
nothing else is showing on the screen. When I access the backend, I get 
the same kind of error at the top of each page but I can see the rest 
too. Any idea how I can fix this ? :

Warning: mysql_fetch_assoc(): supplied argument is not a valid MySQL 
result resource in 
/home/abeditions/www/typo3_src-3.8.0/t3lib/class.t3lib_db.php on line 741

Warning: Cannot modify header information - headers already sent by 
(output started at 
/home/abeditions/www/typo3_src-3.8.0/t3lib/class.t3lib_db.php:741) in 
/home/abeditions/www/typo3_src-3.8.0/typo3/sysext/cms/tslib/class.tslib_fe.php 
on line 2612