In one occassion, this error occurred on a Mac based Pro Tools system. Deleting the Digidesign databases and preferences solved the problem.

Also, the user had some of their session files stored in the Digidesign Databases folders on their external hard drive. This too should not be done. The Digidesign Databases folder is used by Pro Tools to archive Pro Tools files on a each hard drive. Storing your files in this folder may result in errors. You should store your Pro Tools sessions in a seperate folder that you name.