IP changed overnight -- you should be able to find it in the server browser, otherwise add it again as tf2.mooh.org.

Valve updated the player numbers logic so now total slots = 6 + # of spectators.
e.g. if there are 6 players, and 2 specs, the server will show 8/8, if there are only 6 players, the server will show 6/6, if there are 4 players and 2 specs it will show 6/8. So basically that means if it shows a slot free there's actually a player slot available.

One issue with this is if you want to join to spec there are 4 spec slots available but the server browser may prevent you from joining if all the *player* slots are taken. In that case bring up console and type "connect tf2.mooh.org" as before.

If you see the server showing 10/10 that means it has 6 players + 4 specs in which case you won't be able to join using either method. Use rcon and kick someone off :)