This may be operator error or it may be a bug, I cannot find anything that says you can only vote for a single server, only that a server can receive one vote in 24hrs.
I was able to vote for my fav servers earlier this year so assume a bug.
The servers are different types so i should be enabled to thumbs up all three I would think. i started on all three end of February and didn't vote until i had been on them a week. the "already voted" thing started about 3 weeks ago.
One is Mianite custom pvp with a pretty good community.
One is a towny faction server that is closing soon and I haven't voted for recent (Staff great=mod implementation not)
One is a survival PVE server .
especially PVE and PVP are apples an oranges and should each get a vote for being the "best of species"