on the 601. Since pressure at 3000 rpm should reach 3 bar, and at idle 0.3 bar, the relief valve shouldn't open until you are well north of 4000 rpm.

If the problem started after a good rev session, maybe the relief valve is the culprit. It could have opened, and then got stuck shut in such a way as to stop relieving pressure. Hopefully they don't use relief valves of this design on nuclear power plants. Actually, on the picture I see from the 601, they have a guide pin under the piston that looks like it should prevent the piston from getting cocked like yours.

In any case, I think your most effective course of action now is to replace the relief valve. You already have the oil pan off. I don't know if the valve is sold separately, but the pump looks like it includes a new one, and costs $169.