Today this happened again. I was still able to reach the machine via ssh.
There are extra lines in the logging compared to the previous freezes:

Jan 11 11:32:57 nyx kernel: nouveau 0000:23:00.0: disp: 0x000061ec[0]:
INIT_GENERIC_CONDITON: unknown 0x07
Jan 11 11:42:04 nyx kernel: nouveau 0000:23:00.0: disp: 0x000061ec[0]:
INIT_GENERIC_CONDITON: unknown 0x07
Jan 11 11:42:31 nyx kernel: nouveau 0000:23:00.0: disp: 0x000061ec[0]:
INIT_GENERIC_CONDITON: unknown 0x07
Jan 11 11:47:18 nyx kernel: nouveau 0000:23:00.0: disp: 0x000061ec[0]:
INIT_GENERIC_CONDITON: unknown 0x07
Jan 11 13:08:12 nyx kernel: nouveau 0000:23:00.0: disp: 0x000061ec[0]:
INIT_GENERIC_CONDITON: unknown 0x07
Jan 11 13:10:14 nyx kernel: nouveau 0000:23:00.0: disp: 0x000061ec[0]:
INIT_GENERIC_CONDITON: unknown 0x07
Jan 11 13:49:37 nyx kernel: nouveau 0000:23:00.0: gr: TRAP ch 14 [00fdb51000
Xwayland[2381]]
Jan 11 13:49:37 nyx kernel: nouveau 0000:23:00.0: gr: GPC0/TPC0/TEX: 80000041
Jan 11 13:49:37 nyx kernel: nouveau 0000:23:00.0: gr: GPC0/TPC1/TEX: 80000041
Jan 11 13:49:37 nyx kernel: nouveau 0000:23:00.0: gr: GPC0/TPC2/TEX: 80000041
Jan 11 13:49:37 nyx kernel: nouveau 0000:23:00.0: gr: GPC1/TPC2/TEX: 80000041
Jan 11 13:49:37 nyx kernel: nouveau 0000:23:00.0: fifo: read fault at
000725c000 engine 00 [GR] client 0a [GPC0/T1_3] reason 02 [PTE] on channel 14
[00fdb51000 Xwayland[2381]]
Jan 11 13:49:37 nyx kernel: nouveau 0000:23:00.0: fifo: channel 14: killed
Jan 11 13:49:37 nyx kernel: nouveau 0000:23:00.0: fifo: runlist 0: scheduled
for recovery
Jan 11 13:49:37 nyx kernel: nouveau 0000:23:00.0: fifo: engine 0: scheduled for
recovery
Jan 11 13:49:37 nyx kernel: nouveau 0000:23:00.0: Xwayland[2381]: channel 14
killed!
Jan 11 13:50:50 nyx kernel: nouveau 0000:23:00.0: Xwayland[2381]:
nv50cal_space: -16
Jan 11 13:50:50 nyx kernel: nouveau 0000:23:00.0: Xwayland[2381]:
nv50cal_space: -16
Jan 11 13:50:50 nyx kernel: nouveau 0000:23:00.0: Xwayland[2381]:
nv50cal_space: -16
<this last line keeps repeating sometimes several times per second>