[T77911S]

if you are pulling air from another room, you are putting a vacuum on that room, i bet the doors dont need closers. so, you are putting a load on that system. it is trying to draw air into it, and it is providing air to another room.

the server room. this unit is icing because it has cool air going into the evap. the system is deigned to remove heat from the air, so if air going into the system is ALREADY cool, there is effectively no heat load on that system even though the room is getting warm. with the unit running and no heat load, it will/may ice up.

you may connsider just running another supply and return from where you are getting the cold air now to cool the server room and use the server unit as a backup. set the T stat to auto and about 5 degrees higher than what you want and as needed, itwill come on.
i dont know what type your big system is, but you could even put a damper controlled by a Tstat in the server room to keep it cool. keeping the server ac sys off would save the system and money.