I can confirm (somewhat) similar issues with the newest Debian 11 (Bullseye) and Kernel 5.10 on HP Proliant DL380 G7 servers. Boot does sometimes work, then it doesn't work (with a freeze caused by NMI), leading to a very unstable OS.
After disabling hpwdt (module blacklist), the problems were gone. I documented the troubleshooting and solution on my blog: https://www.claudiokuenzler.com/blog/1125/debian-11-bullseye-boot-freeze-kernel-panic-hp-proliant-dl380 Ubuntu seems to have disabled/blacklisted the hpwdt module since 2015 already: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1432837 Maybe something Debian should also consider?
