Rohde & Schwarz NGRU 35 OVP

[Ghitza]

Hi guys,
I really need your help, please.
I've bought this power supply a month ago. I've used it without problems till today.
I cannot set more than 12V in it because the over voltage protection is activated. Sometimes when I power off it and than
power on again, I can set maximum 15V and without any consumer after 1-2-3-4-5 minutes just goes in OVP.
I've tried all the possible combinations of the switches and digital potentiometrs from OVP but absolutelly no change.
It works without problems with maximum 12V with stability on voltage and current.
What I did last time I used 2 resistors of 4 ohms/ 100w in paralel on it to check the ripple on different currents and voltages.
It worked without problems and after I finished I simply turned it off. That was on Sunday and when I powered it on today
this was the "surprise".
Any idea of what could had happened?
Thank you very much.