One of them is a Q+ (ruby edition)
The seller stated, that there are some issues with the encoders, because it wasn’t used for a while. He referred to a statement on the Waldorf page, that says this is a known issue and  can be solved easily by turning the encoders fast 20 times to the left and then 20 times to the right.

When I bought the synth I was testing this. At first all the encoders were jumping randomly when turned. After a while I noticed a big improvement. So I was sure, I could get my 'new Q+' back to fully working by just spending some time.

Back home I realized, that the encoders get better to e certain point. But then, even turning the knobs 120 times to each side does not get them to work properly.
I took off the caps and saw, that someone already put some kind of oil on all of the knobs. I read in forums, that this was a bad idea.

Does anybody of you have an idea what else I could try besides replacing the encoders. I would like to prevent this, because there are 59 of them. All of them show this behavour.