error creating new site

Hi

I have something weired on new site creation via admin pannel (already registered) (3.2 multisite without buddyopress) :

Warning: fopen(/home/mgr/public_html/wp-content/plugins/EMPTY) [function.fopen]: failed to open stream: No such file or directory in /home/mgr/public_html/wp-includes/functions.php on line 4339

Warning: fread() expects parameter 1 to be resource, boolean given in /home/mgr/public_html/wp-includes/functions.php on line 4342

Warning: fclose() expects parameter 1 to be resource, boolean given in /home/mgr/public_html/wp-includes/functions.php on line 4345

Warning: Cannot modify header information - headers already sent by (output started at /home/mgr/public_html/wp-includes/functions.php:4339) in /home/mgr/public_html/wp-includes/pluggable.php on line 934

Any tip to find where does thos come from ?