@JohnFromGWN - Thanks for your reply! I actually use both RPH and Script Hook, but the reason I tried to launch the game with RPH was that it has a convenient backup-revert mechanism... which, apparently, isn't worth much...
I tried what you describe and during my last attempt reverted to the previous .exe, then set "block" for the R*GL internet permission in Norton Security. But upon clicking "Play GTA" shortcut the Launcher started nonetheless with the message it cannot connect to the internet, poor child.
I am back to GTA after a very long time, so I actually rediscover the entire field. It was, like, just two weeks of hassle-free play before this update hit...