Hi UweD,
I do not have much experience, but:
-has it been turned off for a long time? without a good rinse before shutting down? Perhaps your plungers are stuck. (also, loose the tubing after the pump to be sure that there is no clog preventing the pumps to turn normally))
- one of the 4 motors is bad? and needs replacement? (you'd need to disassemble the module and check for any anormal resistance on each of the motors)
- the big boardn or one of the two slave boards defective?
- a bad speed encoder?
-also, one of the four motor belts might be worn out?
if you disassemble the module for the first time pay attention to a screw ring that might fall onto the board.
sorry for the english, hope it was understandable and useful.
Regards,
Miguel