Thanks @lassoan. It does appear to have been due to Avast. In the past, I have typically had Avast pop up a warning alert the first time I open a newly installed nightly version of Slicer, but I have always clicked been able to say “Run Anyway” and I think this has created an exception which allowed normal Slicer operation. I honestly don’t remember whether something like that warning came up this time, but if so, it appears that the same exception was not created this time. I could open and use Slicer normally, but it seems like it was sandboxed in some way to prevent creation of files in my Documents directory or below. When I turned off Avast temporarily and restarted the Slicer version that had not been able to save, the problem disappeared. I added the SlicerApp-real.exe exception for the new version to the list manually, and the problem has not recurred.
Thanks for your help.