Tested again with 4.20-rc7. System ran for 1-2 days and then froze again.
The error looked different though.
[162840.653595] nouveau 0000:01:00.0: fifo: SCHED_ERROR 0a [CTXSW_TIMEOUT]
[162840.653610] nouveau 0000:01:00.0: fifo: runlist 0: scheduled for recovery
[162840.653621] nouveau 0000:01:00.0: fifo: channel 4: killed
[162840.653631] nouveau 0000:01:00.0: fifo: engine 0: scheduled for recovery
[162840.654013] nouveau 0000:01:00.0: systemd-logind[1383]: channel 4 killed!