Taking the external zookeeper down (I'm just testing, so I only have one external zookeeper instance running) and then bringing it back up seems to have caused the overseer to stop processing queue event.
I tried to issue the delete collection command (curl 'http://localhost:7574/solr/admin/collections?action=DELETE&name=c1') and each time it just timed out. Looking at the zookeeper data, I see
and the qn-xxx are not being processed.
Attached please find the log from the overseer (according to /overseer_elect/leader).