Assorted enemies, such as the witch hunters in "Forefather's Eve", go through the routine, but they don't attack and I can't attack them. They show a health of -1 you can swing at them all day, but they don't even acknowledge your attempts to kill them.
This has happened with entire groups of enemies and some enemies in a group. Very frustrating when it keeps you from completing a quest. So far, I've lost five quests to this problem.

Any ideas, anyone?

I had this exact problem with 1.12.1, try verifying your install on GOG Galaxy, this fixed it for me. You may need to reload an earlier save but maybe not

Bug.

"Verifying the Installation" apparently worked, even though it said that no changes were made.

Yep said the same thing for me. The last two patches have messed up the game for me so I may get into the habit of verifying after each one