Debian didn’t exactly crash, when it booted it did not mount tmpfs on /tmp, this is an in memory file system used for temporary files for speed and efficiency.

     Many hours later it mounted it and this broke systemd that needs access to some of those files.

     I do not know what is causing this delayed mounting of partitions but I had a similar problem with mail when I booted it.  I suspected the newer kernel so I booted off the older kernel, same issue.  Then I installed 5.12.2, the most current, still the same.

     After about four attempts it succeeded (same with mail) and is up and running but until I identify this problem reboots are going to be an issue.  This kind of smells like a Poettering systemd issue but, Googling, I can find no other reports of a similar issue.

     I will continue to investigate but for now the server is back up.