I'm still able to reproduce the problem in the latest dev build.

Update: I found a temporary workaround for the problem. If you disconnect from the server then quickly reconnect, you should be able to select a slot.

(And just as a note I'm using an Apple Airport Extreme router. Also, disabling the Windows Firewall doesn't fix the issue. Here's a DxDiag report from my machine if it's at all useful: http://pastebin.com/K94n900f).

Edit:

Looks like disabling DMZ mode on my router may have fixed the problem. Does anyone else have DMZ mode enabled? Try turning it off.

I'm still experiencing this issue on both the stable and development builds.

I was able to select a class just fine after reinstalling Windows 7. However, the problem returned after a few days. Maybe a Windows update is causing it?