I first got this error after an upgrade, but then tried a clean install an still get the same.
After starting Jenkins, I get messages like these on the console:

----------------------------
SEVERE: Timer task hudson.model.LoadStatistics$LoadStatisticsUpdater@37453745 failed
Throwable occurred: java.lang.NoClassDefFoundError: hudson.model.ComputerSet (initialization failure)
at java.lang.J9VMInternals.initialize(J9VMInternals.java:140)
at hudson.model.OverallLoadStatistics.computeTotalExecutors(OverallLoadStatistics.java:63)
at hudson.model.LoadStatistics.updateExecutorCounts(LoadStatistics.java:188)
at hudson.model.LoadStatistics$LoadStatisticsUpdater.doRun(LoadStatistics.java:226)
at hudson.triggers.SafeTimerTask.run(SafeTimerTask.java:54)
at java.util.Timer$TimerImpl.run(Timer.java:291)
----------------------------

Please let me know if you need more information. Thank you in advance.