InDesign won't save files

Help!

I'm running an 21.5" iMac  with MacOS 10.11.5. I've downloaded InDesign 2015.4. Now it won't save any files - even if they have no content! I get an error message similar to this:  "Cannot save "Untitled-1" under a new name. The file DBTmp 27317-1728341967 is damaged (Error code: 0)."

It won't save changes to existing files either. I've re-launched InDesign a few times, but no difference. Any ideas? Thanks, Richard Wells