After this post the situation has changed a little bit.
Sintratec has invited me to come to Brugg and helps finding the problem. I accepted the invitation.
It was a nice day at Brugg. It was very nice to see the people and talk to them face to face. They give me an impression how they work and what they will do in future.
The most importend thing was that they take a lot of time to solve my problem. After the first inspection they told me, that the printer was build correct and in an profession way. But they also see an unusual behavior at the coupling of the spindle to the stepper motor.
When you rotate the coupling by hand you could see e very small (less then a hair) rising of the washer between coupling and ballbearing. This behavior was not normal. The conclusion was that my bad spindle has made a bearing failure at the stepper motor. We changed the stepper motor and made a test print.
The test print was significant better and from my sight of view the problem was solved. With the ticket system i think we will not be able to find this kind of problem. You have to know where and how to look to find this problem. It was a very good solution for me and think for Sintratec too, that i was gone to Brugg.

IMG_0029-2.jpg

- the right cube was printed with the bad spindle
- the middle cube was printed with the new spindle but bearing failure at the stepper motor
- left cube is the final print

You are able to see the wobble every 2mm (one rotation of the spindle) but with the final print you are not able to feel the wobble any more. What you can see is the gear backslash of the stepper motor.