I am on MacOS, I was able to produce this crash in the vanilla launcher with a group of other mods, and in Multimc with only RC and IvToolkit installed.

Forge version is 13.19.1.2188

net.minecraftforge.fml.common.LoaderExceptionModCrash: Caught exception from Recurrent Complex (reccomplex)
Caused by: java.lang.NoClassDefFoundError: Could not initialize class ivorius.reccomplex.RCConfig
at ivorius.reccomplex.RecurrentComplex.preInit(RecurrentComplex.java:111)

Full Crash log: http://pastebin.com/y8tcdtkd

The crash occurs with forge versions later than 0.2172

Also getting this crash with ONLY Recurrent Complex and IVToolKit loaded.

crash-2016-12-09_21.12.33-client.txt
Same crash. This is a new installation of RC, so I didn't have a config file. Before it crashed it managed to create an (empty) config, and then kaput.

Should be resolved in 3094c9e / 1.2.5.1.