When server has above 198 players, it kicks some to make place for server admins. People in the character selection screen does not count towards online players, so when you see about 197-199 players online, you will most likely not be able to log in.
The server has some reserved slots and you usualy can't connect once the server has reached 195 players.
You can try to connect for a few minutes, then you most likely get a spot.