Compability with Spongeforge
I'm not sure in which version this issue happens, but between version HammerCore-1.12.2-2.0.6.8.jar and HammerCore-1.12.2-2.0.6.13.jar the compability with Spongeforge stopped. To my knowledge it seems that HammerCore has edited a class before Spongeforge was able to transform it, caused by HammerCore loading too early?