cron error : rules.variables.inc

Hi,
Since several days, when I run a cron, i have a white page with this sentence :

Fatal error: Call to a member function update() on a non-object in /home/myname/website/sites/mysite.com/modules/rules/rules/rules.variables.inc on line 346

when I get back to the home page, I have this message error :
# warning: include_once(./sites/mysite.com/modules/rules/rules/rules.rules_defaults.inc) [function.include-once]: failed to open stream: No such file or directory in /home/myname/website/sites/mysite.com/modules/rules/rules/rules.module on line 955.
# warning: include_once() [function.include]: Failed opening './sites/mysite.com/modules/rules/rules/rules.rules_defaults.inc' for inclusion (include_path='.:/usr/share/php:/usr/share/pear') in /home/myname/website/sites/mysite.com/modules/rules/rules/rules.module on line 955.
Interestingly, i don't have the rules.rules_defaults.inc in the .../modules/rules/rules/ folder.
I deleted the rules folder and uploaded a fresh version of this module, but the error is still here.

In the admin/report/status, it is write that the cron didn't run since several days.

Any idea ?

Benjamin