I have problems connecting to a NRF51802 via stlinkv2 and openocd. I also tried a black magic probe flashed onto the blue pill board. Both did not work. I have ordered a NRF51822 module which works flawlessly with both devices. So it seems like this is an electrical issue. the NRF51802 board has a 10K pullup resistor on SWDIO and a 500 Ohms pulldown on SWDCLK. I was wondering if the 500 Ohms pulldown might be the issue. Before I try to desolder or cut the trace: Is this a reasonable assumption or is there something else that I might be missing?
It depends on the programmer. 1 k pulldown on the SWDCLK line is recommended btu down to 500 ohm can be ok of the programmer can handle the drive. I recommend using a larger pulldown.
Remove both resistors. The nRF5x already has internal pullups on those pins.