I'm on a server that recently added Refraction. Once we added it, anyone who opens the mod's creative tab crashes their client. All other tabs work fine. This only occurs in multiplayer, so it might be an issue with one of our plugins, or sponge itself.

Crash log: https://pastebin.com/2cLaLVdJ

Minetweaker is causing the issue. not refraction.

I found the problem - Refraction wasn't on the server, only on my client.