Side note -- the restart problem has perhaps spontaneously resolved.  The server automatically installed September Windows monthly updates and requested a restart.  I initiated an update and restart cycle, and went off to make coffee while humming "Keep on the Sunny Side."  When I got back, the system had restarted, apparently with no problem.  Not clear if the original problem had any connection to the r420 hardware, or firmware, or outdated bios.  Perhaps it was all just unknowable 2012 Server bugs, introduced in the August updates and accidentally fixed in the September updates.
My PowerEdge r420 suddenly developed a problem yesterday where it can't be restarted from within Windows (2012 Server operating system.)  If a restart is triggered, either by the user or Windows Update, the server seems to shut down instead, which is a major problem for a remote data center machine. This type of problem seems to have hit various 2012 servers with various hardware over the years, and never did any case seem to be truly understood.  But sometimes it's blamed on outdated BIOS or firmware having become incompatible with the latest Windows updates.  In my case, KB4512488, the August quality rollup, is the most suspect recent update installed.  I wondered if any other r420 owners have encountered this problem, and if any causes have been identified.
PowerEdge r420 won't restart, shuts down instead (Windows 2012 server)