Same issue here.
Downloaded and setup brand-new Tekkit Server, tried to connect right away with using Tekkit in the launcher. Gave me just the white loading screen for 5 minutes before quitting.
Ran my Technic on the launcher, it updated, tried again with Tekkit, game loads, but when trying to connect to the server, I get the same message:
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Disconnected by Server
This server requires you to have Minecraft Forge installed.
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Tried to connect with Technic at launch, no change.
Tried to connect with Vanilla, no change
Tried to connect with Vanilla and updated minecraft.jar with the latest Forge Client (1.3.3.12), and it remains at "Connecting to the server..."
Any updates on this situation?