I have been using Rules with custom Workbench moderation states to notify users of a status change. Occasionally I would see a PHP Access Violation error, that would go away. I finally nailed down a consistent way to reproduce the error and a way to correct the error. I was installing some mail modules (Mailsystem and mimemail). I finally found that if I exported one of the rules, then imported without the if statement it would work, then add in a moderation rule for draft status. Strange thing the existing rules started working.
I found when you installed a module you can see the php access violation error, when I try to edit a rule with a custom moderation state. I have a rule for the draft state, it fails the first time but the second time I edit it works and the custom states work as well.
I think that the custom states do not get loaded by default, but if you load a draft state it resolves the issue.
I am not sure if it is on the Rules or Workbench side so will post in both issue queues.