I just recently upgraded my Plesk panel from 8.4 to 9.5.4 and it broke all of my Drupal sites that shared the same code base.  So all of my Drupal multi-sites were down except for the base install.  I had about 35 sites down for almost 2 days until a friend of mine figured out what was wrong.   All he did to get the sites back up was to goto the php.ini file and disable safe_mode!  Yup, I spent almost 2 days racking my brain trying to get the sites up and running and he figured it out in less then an hour! :)

Goto your php.ini and make sure safe_mode = Off and hopefully that'll work if you're experiencing the same problems with Drupal/Plesk.

safe_mode = Off