It seems i’m experiencing this PHP bug and by reading all the feedback at that page, it seems someone else is experiencing the same as well.
Oddly enough, the status of the bug tracker is still showing a “No Feedback” after three years of reporting: anyway, i’m experiencing this while not being under an heavy load, and i just added this line to the interested .php file just before the session_start:

ini_set( "session.save_handler", "files" );

This way it seems to fix the error i was constantly receiving for 10 minutes, but i’ll have to wait some more i think.

<edit>
Well, it seems it’s working correctly now, i’m not experiencing the problem anymore.
</edit>