Error - "Invalid class found in file" WTH???

Went to open a part this morning, only to get this message:

"Invalid class found in file"

and Solidworks would terminate the "open file" command.

I believe it is non-recoverable. Has anyone else experienced this?What is the cause, and is there a way to recover? Is there a way toavoid it in the future?

We have tried opening it on several different machines, makingcopies and opening, opening as lightweight, view only. No dice,same message.SolidworksGeneral