Special thanks to Oleg Nenashev for detecting my mistake. The issues I was seeing were caused by
my mistakes and are not related to anything specific to 2.204.1-rc.
I made a merge error in the definition of a Blue Ocean organization folder and deleted an opening tag in the 'scm' section of a pipeline library definition. Once that merge error was reverted, the job loads correctly and completely in Jenkins 2.204.1-rc.
The on-disc copy I was using at run-time was different from the copy stored in source control. Once they were made the same, then Jenkins 2.190.3 showed the same failure.
The second mistake I made was that I had omitted a JCasC agent definition for one of my Windows agents and was mistakenly using an on-disk copy of the agent definition. The on-disk copy was damaged in some way. Once I defined the agent in JCasC, the agent ran correctly with Jenkins 2.204.1-rc.
Sorry for the false alarms on both items!
Mark Waite