I spent quite some time trying to understand this "greyed join/getmod button".
I haven't been able to identify a pattern: sometimes it works, sometimes it doesn't, seemingly random.
Waiting as Tim suggested a few times doesn't seem to work either.
For now, the only workaround that *seems* to do it for me is to quit the lobby and restart it again from the launcher.
Doesn't seem to be linked to any particular track.
I also tried setting up my own dedicated server, with single and multi track mods, mods built with 382 mas2 tool and built with 494. No difference. Tried to access my own server from my own network and from a remote network. It worked all the time ... :-|
I also tried to dump network traffic from the lobby application. What I see is that the MM server returns the correct information on active servers. BTW, I haven't seen any parsing CTD problems anymore. Then when you click on a particular server,
the lobby app makes a request to that particular server for session information.
That is the point where things can go bananas.
When it works, the rf2 server (not the MM server) returns back the session info just fine, and lobby app displays all the information about it, including the getmod/join button (active).
At times though, the data from rf2 server is never seen by the lobby client. Reason unknown though. My theory was that the rf2 server wasn't reachable, or maybe it was crashing or something. However, it's not clear this is a server problem. Might just be that the lobby client is doing something wrong, because (AFAICS so far) quitting lobby and restarting seems to be a good workaround.
Last thing I noticed is that when you select a server once, and no session data is received back, not only the getmod/join button is greyed out, but
lobby app will continue issuing session info requests every 0.4s-0.5s or so, no matter what. I wonder whether this can have any impact on rf2 dedicated servers load. Not all of these failed requests die immediately without reaching the servers though. Some of them actually do get a response, and that is usually a HTTP 404 Not found error. Sometimes this error is, oddly enough, in German.
I'm sure ISI is on top of this, but I wanted to help as much as I can by doing "black box" testing

to get this issue fixed sooner.