Thanks for your advice.
Yes I have done all that.
After further investigation seems to come from my moving the .pov files that
were open last time I used POV-Ray.
The problem disappeared after I removed the related entries in the Windows
registry (HKEY_CURRENT_USER/Software/POV-Ray/v3.6/POV-Edit/Open).
Guess this can be qualified as a POV-Ray bug?