When starting a non-clustered PRO instance (ie. nexus.clustered=false) it is still possible to get the following warning in the UI, which looks HA related:
This warning comes from the DatabaseQuorumReset JS controller, specifically when the NX.direct.node_NodeAccess.nodes query in its "init" method fails.
So far I've only been able to recreate this when stopping, or very occasionally when rebooting, the NXRM instance while the UI was loading - or by disrupting the network connection between the browser and NXRM while the UI loads. (Note the local node name is used when resetting the quorum to ensure we reduce it down to just the current node.)
The DatabaseQuorumReset JS controller is always activated regardless whether the current instance is clustered or not. I believe this is to let people reset quorum even when the instance is non-clustered (there are some edge cases where it's necessary to boot a node as non-clustered to get NXRM up and running, so the DB can be fixed in preparation for running as clustered).
Given the difficulty in recreating this issue, we might decide this ticket is enough to document that this message can occur even in non-clustered mode - and more importantly it doesn't indicate the instance was ever run in clustered mode.
Alternatively it might be possible to avoid this issue by lazy loading the local node name, rather than eagerly fetching the status when the UI loads. But is the extra complexity worth it?