Need help with a hand-wired split keyboard with kmk

I built a split bastardkbskeletyl and used kmk as firmware on rp2040-zero.

The split module is added last and this is the config for split:

split = Split( data_pin=board.GP11, split_side=varList.ss, use_pio=True, )

I originally used GP10 on both sides... just switched to 11 as I thought maybe the pin of the mc was broken... but after a few minutes, the right side, slave (not the usb side) stopped working again.

When I plug in usb to either side alone, I see debug output, I think it is really nicely wired and either side works nicely alone...

Any ideas? It's kind of weird.. I know GP11 is not a UART pin, but that's why use_pio is True... and it worked again for some minutes. I think it hangs up or so.