Unfortunately it seem to be the contacts inside the switch, which are the problem, because the fault came back. The sony drive looks like new, probably because it was in an fanless external case and never hang in the constant dust wind of a computer. The pin itself is easy to press down.
I found a post in (I think macrumors), where someone stated that normally some compressed air and contact spray would help. Tried that today, still no luck. Sometimes it works, sometimes I have to eject, press down the pin with a spudger, reinsert and it would work again for one or two floppies.
In these rarer cases the guy said, the only thing, that helps is to cannibalize another defect floppy drive and get a SW1 (the name of the switch on the board) from there. Unfortunately I neither have a spare switch nor am I very good in soldering.
Anyone here on the forum, who knows more about implanting a new write protection switch into one of these marvelous mechanical pieces of art? And maybe a source for these switches?