You could try a clean re-install of the game and patch. That may help if the problem is something in the game itself, rather than the save.
Zip or otherwise backup your saved games folder to another location first, just in case. Each save is a separate subfolder in the '..\Beyond Divinity\savegames' folder; the 5 'bd_act#_start' folders are created/updated by the game/patch, and do not need to be backed up. The folder names may be different in the German version of the game, but should be similar.
Un-install the game, re-boot, in Windows Explorer browse to the 'Beyond Divinity' folder and make sure there are no files or folders remaining, except within the 'savegames' folder (though the 5 'bd_act#_start' folders should be gone). After that, shut down all non essential programs (especially anti-virus), re-install the game and then re-boot. Shut down all non essential programs again and install the patch, then start the game, load a save at the end of act 3, enter the academy, save at the start of act 4 and see if you can load that save.
If you have access to another computer, you could try installing BD on it and loading a save at the end of act 3. If there is some kind of conflict with your system and something the game does on act transition, or a problem with the game that reinstalling does not help, then trying a save on another computer may work.
If neither of these suggestions help, then the problem may be something in the save folder itself.
