Here is the USB-C PD spec for maximum power:

I had the maximum current set to 5A and testing with my power bank failed, because it can only deliver 5V or 9V with 2A max. Reducing the max current to 2A made everything work. Of course if I request higher voltages, negotiation still comes up with a disabled power switch, which makes sense now. I received a USB-C PD wall power supply in the meantime and it can and does deliver 20V at 5A and everything in between. Testing is done in very short cycles because my 100W 4 Ohm test load heats up really fast. I need a bigger heat sink. Also thinking about using light bulbs as load, what a bright idea.