Thanks Jeff. There's something odd happening because NOTHING I did deliberately could cause SF suddenly to lose track of where the debugger is.
When I had problems earlier and re-installed SF3.9, I had to use the 'REPAIR option, and this didn't ask for a pathname.
I checked that SF had not appeared in the default directory C:\Program Files (x86)\Simply Fortran\, because that's not where I put it. I always install in C:\Program Files (x86)\FORTRAN\SF. It had not re-installed in the wrong place.
What I've just discovered, though, is that the debugger paths seem to have been re-set, to
C:\Program Files (x86)\Simply Fortran\insight32\bin\insight.exe
C:\Program Files (x86)\Simply Fortran\insight64\bin\insight.exe
None of the other paths have changed in options, only the debugger paths.
I have had to change these back, manually, to,
C:\Program Files (x86)\FORTRAN\SF\insight32\bin\insight.exe
C:\Program Files (x86)\FORTRAN\SF\insight64\bin\insight.exe
This correction has got the debugger running again.
Thanks Jeff.
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John