Hi @haenno, is the rig stable without OCs? (always worth checking after adding new GPUs if you have problems, it rules out bad OCs and also checks if hardware & power are working ok and you don’t have any PCIe lane issues). Be aware your power consumption will increase substantially with no OCs so double check your PSU and PCIe cables have sufficient power capacity.
Also you can try removing the SoC settings (Freq & VVDmax) for GPU6 and see if it is stable then start to reduce. The hashrate for GPU6 is quite low, it might just be silicon lottery or it might point to something else the rig is not happy about.