Agents not connecting to OpsCenter via STOMP, UI displays question marks in the STOMP column

The previous DBA created and connected a cluster (4 nodes) to DSE OpsCenter. It's almost perfect (nodes are connected and visible) except for the STOMP service. In the OpsCenter -> Nodes -> Agents there are question marks in the STOMP service column. What should I check first? The firewall seems to be ok, like in the other 3 clusters, which work perfectly fine.