Getting error message that course is in use but it is not.

I keep getting the a message stating "The file "x" is beig used by another process or you do not have the proper security permission to access this resources.  Make a copy of your project file, then try to open the new file."  I am the owner of the file, so I should have the proper permission.  I have double and triple checked to make sure all my windows are closed and the course is not open in another tab. If I do create a new file, the new file will open, but if I close the file then try to reopen, I get this message again and have to create another file.  I really don't need 20 copies of the same course (additionally, this puts me at risk of opening and editing the wrong copy or deleting one that I want to keep) - any suggestions?  This happens frequently and is occurring with multiple files.