Might have a memory leak when running ckproxy. Had a Pi start acting up this morning after running fine for about 3 weeks. Turned out it ran out of free memory and the watchdog had trouble killing it and killed some system processes instead. Checked the other Pis and the ckproxy process was using high amounts of memory. Percentage-wise the memory on most of them had gone over 60%, where on a fresh start it's more around 6-7%.

Edit: To note the Pi that locked up was also the one handling the highest hashrate, so that may be related. Since it ran out of memory while the lower hashrate Pis still had some free.