Quote from MoffKalast
There were others with the same/simmilar problem, but they usually had installed the guliver mod which has known incompatibility. Are you sure this is the only mod you're running?
It's nowhere near the only mod I'm running. I've got most of the mods from FTB and several others installed in a custom 1.5.2 setup, the only ones I can think might cause the GUI to not open are Archimedes boats, small boats and Inventory Tweaks. However I can't figure out why this would only happen in SMP when it works fine in SSP since the Server/Client sides are supposed to be virtually identical.