I've been tracking this error for a couple of months now and I have isolated to just your mdecore library v1.8-1.2.5

The only mod loaded and here it is: http://paste.ee/p/Vj50b

When the rest of my mega mod pack is loaded, this one error appears to cause an cascading effect that prevents the client from getting and further generates strange netty login issues and index out of bound scenariors.