As I detailed on Jenkins IRC last Friday 2020-06-05 (log at https://echelog.com/logs/browse/jenkins/1591308000 - posts by "jimklimov"), I updated our instance from weekly 2.238 to 2.239 and hell broke loose with Jenkins not finding various classes (I think different ones at various restart retries), always mentioning a lot of problems with QueueItemAuthenticatorConfiguration - and not starting queued builds, trying for 10 minutes but not seeing SSH Agents (while Swarm ones did get connected - but not used) etc. Sometimes it had not loaded the base classes such as Configuration and FilePath and failed to start meaningfully. At some point during restarts, several tags from config.xml disappeared...
Reverting to 2.238 core that worked (albeit with the updated plugins from that week's brew) I could not restore the server - it failed with similar errors.
Ultimately, looking for other similarly worded issues I followed the ideas from JENKINS-61990 and moved away the saved queue and milestone XMLs, and starting from clean slate in this manner worked. However, a clean start in 2.238 WAR and restart into 2.239 failed like in original issue. Cleaning the configs and starting 2.239 worked well. Safe-restarting 2.239 also seems to succeed with the XMLs it had saved.
So initially I wanted to log this as a blocker but in the end a workaround exists so it is not exactly such (assuming lost queued builds can be tolerated). Also, in https://www.jenkins.io/changelog/ weather everyone else seemed satisfied with this release.
For the agents' connections, the fuller stack traces are here : https://pastebin.com/siJbMdWP and a lot of (probably from UI driven status refreshes?) same class in these combos of lines: https://pastebin.com/MFvQukr3