I am getting a Stack Overflow error in my MultiMC Console when Java crashes MC straight to desktop. I am running Natura 2.1RC on Minecraft 1.5.2. It says 2.1RC in MultiMC but it is 2.1.5.

This happens only in the Nether. It seems that no Wither Skeletons are spawning in the fortresses either.

There is no crash report to be given, neither from Minecraft or a crash report.