If you see something like this (Could not connect to Swarm at)
It is possible that your Content Gateway/ Service Proxy has been upgraded to 5.2.1 but your Swarm UI has not been upgraded to the minimum required version that is supported by 5.2.1.
There is a new Swarm 9.2.1 bundle (marked with a -2 designation) that includes Swarm UI 1.1. Please download that and upgrade your Swarm UI (which is likely also installed on the Content Gateway). Log out and log back in. That should fix the login issue.