Andrew,
GG was able to reproduce via:
#create a session with a "Disolve" transition; save backup; quit
cd /path/cinelerra-5.1/bin/plugins/
mv video/dissolve.plugin /tmp/.
#restart cin5, and load from backup
#Notice the message in the controlling terminal, same backtrace as dump
# [Detaching after vfork from child process 16241]
# The transition 'Dissolve' in file '/root/.bcast5/backup.xml' is not part of your installation of Cinelerra.
# The project won't be rendered as it was meant and Cinelerra might crash.
He has made a mod that reports the problem of the missing plugin_server plugin BUT it might not be checked into GIT for a day or 2.