I wonder if it is a graphics driver update? I occasionally get this problem or flashing screens. When I update the driver it goes away. Just a suggestion.
No, that's not it. I wasn't sure before, but now I've tested it more. It seems to be a problem with old saves from the previous version. Not all of them, but they seem to be the ones after the first freeroam part of the game.
Yes, I noticed the note about old saves not working in the OP, but I wasn't sure how old that note is and if there could be a way around it. Usually there is.
The thing is, I extracted *.rpa files using Unren when I tried the new version the first time.
Now testing the original without extraction, it seems that none of the old saves are working. The old known rollback error that comes to haunt you if rpyc files have been re-created from scratch by the dev and the rpyc indexes don't match with the older version where saves were created.
It seems that extracted game is able to load some of the old saves, but most of them are dark and some will also throw the rollback error. You can get rid of the Ren'Py rollback error by copying game script *.rpyc file where the error occurs (script.rpyc in this case) from the game version where the save was created, to get the original rpyc index for the saves, so that Ren'Py will re-compile the new *.rpy file version with the same name (script.rpy in this case), but use the old rpyc index for it to keep save file compatibility.
Doing that helps to get rid of all the rollback errors, but then almost all of these old saves are dark, as if a Ren'Py image matrix transformation "night filter" has been applied over everything. Weird, haven't seen anything like this before. Don't know how to debug it or find the source of the problem without comparing all the script files with their old versions...
I guess there's no easy way around it, have to really start to play from the beginning. Oh, well, I don't remember much about it anyway.