Sorry for not answering since the last weeks.
The solution was really the HV-Contacs on the left side at the place for the cyan cassette.
The lower contact-spring had no good electrical contact to the cassette. I fixed it and now the printer works without colored stripes.
THANK YOU very much for your technical support! :)
BUT:
the calibration doesn't works properly.
Black and cyan are calibrated, but magenta and yellow are not congruent with cyan and black. They are differing horizontally about 1 mm (~ 1/20 inch).
I did calibration from the menu of the printer. What can I do instead?