I have received the Bonus DVD yesterday. As it seems, using the versions of the games on the DVD has solved my problems:
I completed "Situation: Comedy" yesterday, saving, exiting the game and reloading several times along the way. No problems.
I have also tried to start Eps. 3 and 4 and they, too, seem to be working now.
Now, I'll enjoy the rest of Season 1 and let you know if there are further problems :-)
Thanks for your help,