I had a wacky error the other day on saved InfoPath forms. This prevented edits and approvals.
-Is invalid. It may refer to a nonexistent file or folder
The cause of this error is because the SharePoint Designer workflow modified the item, with a published workflow saved with a Forms authentication ID, rather than an AD ID.
After a lot of searching, this error can be easier prevented with publishing workflows with the AD ID.
So what do you do with existing form submissions? Download the InfoPath XML file, delete the file in the library and re upload it.