Got an update for those who responded to my (admittedly emotional) request for help:

Apparently, Windows security (or one of my other protection systems) started flagging saves from Source Filmmaker as unnecessary tampering, and I didn't know this until I tried again this morning, only for the popup message to appear this time.

Needless to say, I clicked the "allow" option. Now I seem to be able to Export > Image again.

I thank all involved for their input, and their patience with my borderline meltdown.