I have tried now different things and today is the day to make some conclusions.
First of all I must admit that I was wrong (sometimes I can be very stubborn) and I want to thank those members, who kicked my ass and showed me the right way (maybe I need this from time to time).
I tried different methods with three Semogue shaving brushes that have been more or less unusable due to the described effect. In fact, it was only some kind of stain like lime soap, lanoline etc. First I tried cleaning with a solution of vinegar and a drop of dishwashing detergent, which showed no effect at the first step. I read later that even small remains of vinegar are a real foam killer, maybe that was one of the reasons.
Then I tried repeating this process while omitting the vinegar at the last step. This showed already some results but the result was not as good as desired. Until now I tried this with two Semogue brushes, a 620 and a 830.
Then I contacted my brother, who is a professional goldsmith because I know that he uses Borax (boric acid) as a soldering aid. He gave me some of this for my cleaning experiments. For my next try, again I used a small bowl with warm water and added a tablespoon Borax and a drip dishwashing solution. Every of the three brushes has been left for half an hour within this solution and was squeezed and dispersed every few minutes. After this I cleaned the brushes thoroughly and let them dry over night.
And now they all work really well again! For my opinion the cleaning with Borax was the most efficient way and showed the best results. Maybe I should think about cleaning my brushes at a regular basis to prevent this kind of pollution.
Anyway, many thanks to all the forum members that helped me. I am happy that I can share my experiences for anyone.
Batou