Fabric Explorer - Cannot expand the tree despite being connected, or doing a reconnect

This could be hard to reproduce.

I have created some containers, and browsed my fabric for a while. Then I created a 2nd broker container. And the tree was sort of resetting. Then the root "My Fabric" node cannot be expanded (eg there is no little triangle). So I cannot expand the tree to browse the fabric.

If I disconnect and then re-connect, then the triangle does not come back.
Neither if I reset the Fuse Perspective etc.

I noticed a little spinning ball when I connect to the fabric.