Reinstalling had no effect, and I didn't think it would since other files were running fine.
I seem to have found the culprit, namely a map entity door. Runs fine with this replaced, but I have no idea why a door would cause that error.
Thank you EpsiloN for the suggestion. I have to say that besides your response I am disappointed that there was no help at all coming from this forum. I think 3DGS is a great product, the tutorials are great, and I had thought there was help available here.