Ok, here's an intermediate result and an obvious root cause found.
I desoldered the switch completely, (quite easily done) and measured all contacts.
Found 2 out of 8 switches which had higher resistance of several Ohms.
Then I carefully pulled out the lever inside a small box, because I ended up with 8 very tiny springs and 8 tiny wipers.
During pulling out and testing different wipers on the faulty contacts, I identified, that 2 of the wipers were the problem, not those 6 out of the 24 stator pins.
The stator pins were glued to the case, at its bottom and at the top plate, with some sort of epoxy, which already showed signs of decomposition.
Inside the plastic case, small pieces of debris could be seen.
The debris originates quite obviously from the epoxy.
The surfaces of the stators seemed clean, as far I was able to look inside.
I could also see some abrasive dust (correct term?) originating from the lever.
So I also cleaned the inside with a cotton bud.
When I looked closer at the gold plated wipers, I noticed a faint film on the wipers, which can be carefully rubbed of with a dry, soft cotton bud.
This film probably caused the bad contact.
Otherwise, there were absolutely no signs of oxidation or abrasion of the gold surfaces.
My explanation for that film is evaporation of plasticiser from the case plastics, or from the epoxy.
A normal process for 25 years old stuff like that..
(I have seen Buzz Aldrins lunar space suit in the Smithsonian's Air & Space Museum, and all the plastics of that historic piece were in the process of disintegration, without any chance of holding that up, as it was mentioned there, I think)
Tomorrow morning, I will clean all wipers, and will try to reassemble those small puzzle pieces. Break-Before-Make wipers and Make-Before-Break wipers.... oh Dear!
Sometimes, small springs spring far away...
If I am successful, I will post a few pictures.
Good night - Frank