I'll try to briefly describe my problem: My hydraulic circuit (the pumps are supplying 130 bars of pressure) consists of a closed-center proportional valve that drives a hydraulic motor (which is used for rotating 3 ton coils in a rolling mill)
I removed the pressure reducing valve (before the proportional valve) that was set to 90 bars in order to speed up rotation time of the platform but after a few days of normal performance on 130 bars the proportional valve lost 'zero position' (spool was shifted around 3%). Since this happened on 2 identical machines I'm quite sure that it wasn't a coincidence. I readjusted the zero but I had to return the pressure reducer and therefore slow down the cycle again. Could somebody give me an explanation what happened?

You may find this article useful. It explains the different reasons behind valve drift.

Rexroth acknowledges a drift with changes in both pressure and fluid temp although not to the extent that you have seen.  I've used that valve before but have had better results from Moog.  If the drift is causing a positioning problem you can always sandwich a "blocking valve" under the prop valve.

I admit I am no expert on these, but I would have thought you originally had a problem in flow...not pressure, you could try a higher flow capacity pressure reducer, keeping your zero & still achieving speeds required.
Increasing pressure tends to have negative effects on many types of valves...again I am not expert on this particular type, but above is what I would be looking at.

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