Diags are after rebooting so we can't see what happened, but most likely and like mentioned it was a controller issue, try using it in a different PCIe slot if available, and if there are more errors grab the diags before rebooting.
Jan 8 13:04:10 Beowulf kernel: EDAC MC0: 1 CE memory read error on CPU_SrcID#0_Ha#0_Chan#0_DIMM#0 (channel:0 slot:0 page:0x17fa6b offset:0x600 grain:32 syndrome:0x0 - area:DRAM err_code:0001:0090 socket:0 ha:0 channel_mask:1 rank:1)
Server is reporting memory issues, System/IPMI event log in the BIOS might show more info for the affected DIMM.
And FYI for the future the parity swap procedure must be done completely without any interruption or it will abort and you'll need to start over from the beginning.
The problem is the other pool member, looks like it dropped offline earlier, probably due to the controller issues, run a scrub on the pool and check that there aren't uncorrectable errors, also good idea to check this.
https://forums.unraid.net/bug-reports/stable-releases/683-shfs-error-results-in-lost-mntuser-r939/
Some workarounds discussed there, mostly disable NFS if not needed or you can change everything to SMB, can also be caused by Tdarr if you use that.
Forgot to mention, there shouldn't be, but if there's an "all data on this device will be deleted" warning after any of the pool devices don't start it, in that case reboot first, warning should them be gone.