I've seen this too and documented my findings here: https://www.claudiokuenzler.com/blog/1125/debian-11-bullseye-boot-freeze-kernel-panic-hp-proliant-dl380
(reason is the hpwdt module) This bug is most likely a duplicate of https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=898336 where the same NMI's are reported with Kernel 4.16 but with Kernel 5.10 the boot issues/crashes seem to be even worse. Maintainers, please consider disabling (blacklisting) the hpwdt module by default (same as Ubuntu). If anyone REALLY needs it, it can be manually enabled.
