This just started happening to me after I set up a parity drive.
I had been using Unraid without a parity drive for about a year, and then I added one a little over month ago. This happened during the first parity check but it finished successfully after rebooting. But now (1 month later), its second parity check is failing with this problem repeatedly (3 times today). I have changed parity checks to yearly until I can figure out how to stop the entire system from hanging.
I have attached a log from the next successful boot, and here are the messages it printed before the crash that led me to find this thread:
kernel BUG at drivers/md/unraid.c:310!
invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
CPU: 13 PID: 4587 Comm: mdrecoveryd Not tainted 5.19.17-Unraid #2
syslog.txt