This can be caused by corrupt Database files. To resolve this you need to delete the files so Pro Tools can recreate new files.To do this, first quit the Pro Tools application, then:

On a Mac:

There will be folder called "Digidesign Databases" at the root level of each volume (partition) on the system. Delete these folders on each volume.

Then go to: Mac HD / Library / Application Support / Digidesign / Databases / Volumes

and delete the contents of the Volumes folder.

On Windows:

There will be folder called "Digidesign Databases" at the root level of each volume (partition) on the system. Delete these folders on each volume.

Then go to: C:\ Program Files \ Digidesign \ Pro Tools \ Databases \ Volumes

and delete the contents of the Volumes folder

C: is assumed to be the system drive. If it is not, replace C: with the appropriate drive letter