When i was talking to someone their server name initially displayed correctly but then suddenly after a few minutes of not talking to them, the next time they whispered it showed a number in place of the server name. They were still on EN2. I also tried whispering myself and my whisper did the number too. What's going on?

Did you go on the forum at the time? It's been linked to that, but never been able to prove it myself.

When you are whispering a player in-game the same time you are logged in into the forum, "100" will be displayed.