Plutonium

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Wdym? Can you elaborate? All you have to do is connect to a server, wait for the mod to finish downloading (do not interrupt it) and its gonna automatically connect you to the server once it's done. Ofcourse it could take some time depending on your internet and your hardware on how much time it takes to load the map.

You probably interrupted the mod download, never do that - let it complete. Anyways, delete that specific mod from %localappdata%\Plutonium\storage\t4\mods and reconnect to the server. It should download the mod again and let you enter without any issues.