After having disassembled that thing into about 40+ parts, washed everything in naphta/graphite,
reassembled and on rare spots applied some grease it seems to work great now.
The most challenging part were many little springs, gears and metal parts that needed to be put
together completely correctly to make all times and the B mode work.
A big help was the thorough photographic documentation of the disassembling with about 350+ images.
This was maybe the most important thing I learned - the value of extensive photo documentation.
I can not strongly enough discourage anyone from trying to fix a central shutter
by soaking it it in naphta or any other solvent.
Its a sure way to make it dysfunctional.
After this experience (I didn't soak it) I can say disassembling and precise washing and reassembling is the way to go here.
Without supervision by someone experienced I'd say its near impossible to have success for a first time attempt.
For anyone interested:
The photo documentation of the disassembling on the public photo album of the mechanic can be found here:
http://fotoalbum.web.de/album/CameraLensBi...ekor3-5-90-6312(The link is dysfunctional at times, since web.de seems to have issues in the moment)