thank you all for valuable hints.
As a result I see, that no cleaning will be the best, waterbased cleaning may be risky and highly volatile substances were sometimes used.
I have some plates of low picture quality for testing and will try 95% ethylol first, thereafter maybe acetone, which is easy available.
The plates were not retouched (I know their complete history), most are boxed and Quality is good (the photographer worked in Zeiss Göttingen factory).
Cleaning will be necessary for a smaller part of them
best regards
Hendrik