Well, after a very long time of having no problems with the forum on the Interserver host... one day it broke. And after two days of fiddling around trying to fix it, and failing, I decided to pull the trigger and update the forum to the current highest version that still works with .net framework 4.7.2, which, after considerable additional fiddling and troubleshooting, now works fine. Great. All is well with the world of Elthos once again.