Drupal unexpected "Fatal error: Class name must be a valid object or a string [...] common.inc" on every page
The error means that Drupal can't find the entity type information for a entity that it tries to load.

Add var_dump($entity_type); in there and reload the page. The last one that it's going to print (if there are multiple) is the culprit. What you need to do then depends on the entity type. It could for example be that you disabled a module which is still being called.