Under certain clustering conditions (apparently when the role of 'master' cluster node has been moved to another node), nullpointerexceptions can occur that relate to MUC functionality.
An initial investigation and possible patch was provided by community member CHP in https://github.com/igniterealtime/Openfire/pull/1521 https://github.com/igniterealtime/Openfire/pull/1748 https://github.com/igniterealtime/openfire-hazelcast-plugin/pull/55 and https://github.com/igniterealtime/Openfire/pull/1749 - but as of yet, they don't appear to fully rectify the problem.
Steps to reproduce the problem in the thread:
Create a cluster
Connect to a MUC on the junior node
Switch off the junior node
Connect to the MUC on the senior node
(This is not the only problem described among the tickets linked, but may continue to show problems once #1748 is fixed)