Glad to tell, that I found a solution to this issue (at least concerning my probs). Maybe the patch didn't work so well after downloading the first time.
I did a "check on the gaming data" in the locale data section (as shown in the screenshot provided in the first post - last option). Some files seemed to be corrupt after the patch and were renewed with this option by the system via downloading just these files anew. Afterwards I could start the game and my savegame without problems (at least until now), except for a little delay in loading. But I'm happy to be able to game on without loosing all my progress.
Maybe this post could help some gamers facing this or similar issues.
Sebastiano's post in Savegames are here,but all characters,items and infos - gone... was marked as the answer