I have a user getting the error "Warning! Available window resources are critically low. SOLIDWORKS cannot open any more windows. Close some open SOLIDWORKS documents.

It appears that he has an assembly and a drawing open and that's it.  He is using less than 50% of RAM  and 0% of his CPU.  This seems to be an error in Solidworks.  Does anyone know why this error pops up?  Google is of very little help.