Well, I'm making some progress, but also have some more questions.
Reading some other topics on this forum I saw that the thread should go around the lower tensioner twice. That made sense to me because the lower tensioner consists of a wheel sandwiched between two felt discs. The thread just slides over this wheel and the tensioner is not working at all. Going around a second time with the thread may improve the grip on the lower tensioner and I may slacken the top tensioner. Which may solve the problem of the unwinding thread.
So far the theory
In practice I had to go around the lower tensioner wheel twice more ( that makes 2 and 3/4 turn) to stop the thread from sliding over the wheel. But as soon as I increase the tension, it starts sliding again. And to get a proper stitch, I still need a lot of tension on the top tensioner.
So I figure the bottom tensioner is not doing its job, but all my 45K's are like this (last week I bought a third one, a 45K58). A different type of thread (or yarn?) probably will solve the problem, but still I would like to see the bottom tensioner doing what it is supposed to do.