MarkoH01: It's definitely worth to try to patch the game and see if the savegames will work again. Afaik the bug also does not
affect manual saves made before but it will overwrite the automatic saves so if you still have an older manual save this may work as well. It's also a possibility to patach the game and the use console cheats to quickplay through the game until your current state. It's not the same but it may be a way to play the game further.
yogsloth: OK, thanks, but I need my hand held.
I can't figure out how to do it.
I re-downloaded the DLC, but when I try to install it, it says the base game is not detected... which of course is not so.
Do I have to reinstall the entire game? Should I un-install it first?
Yes, that is what I would recommend. The safest thing you can do is to secure your (even if corrupted) savegames. In case you are using Win7 (I think Win8 is the same) it would be here:
C:\Users\USERNAME\AppData\LocalLow\Daedalic Entertainment GmbH
Just copy the folder anywhere safe. When you uninstall any GOG game you will be given the opportunity to not delete your savegames but better be safe than sorry. After securing the savegame folder uninstall the complete game and reinstall the game with the newest GOG version. After installing the game reinstall the DLC.
GOG has updated the game as well as the DLC on the same day so it could be possible that you HAVE to reinstall the game again. If you still need help just post or write me a chat-mail - I will try to help if I can. Good luck!
Edit: And if you like you can send me the files in your savegame folder (just upload them to a fileserver like zippyshare.com) and I will try to get them to work.