CloudBees has upgraded the JGroups dependency for the CloudBees High Availability plugin, which means instances with JGroups customized through the GUI fail to start and existing jgroups.xml files may no longer be compatible.
Users with instances using the CloudBees High Availability plugin with JGroups customized through the GUI (under Manage Jenkins > Configure System > High Availability Configuration) must be updated to 2.249.2.4 or higher.