Problem joining lobby

I am attempting to join the lobby for the Labour Party Conference 2021. I received the invitation. When I click on Join Lobby, it says I am not logged in to the right account. (I only have one, so I re-signed in with the same one). It shows a column of chat which I can contribute to, but the main window panel is blank except for a message  "Your lobby is being prepared. Please wait a moment" but nothing ever happens. The client said it needed updating to 5.7.6 but it's already 5.8.3 on WIndows 10 20H2.

I successfully used Zoom on a different connection yesterday, so the basic function is working.

How do I get into the lobby properly?