it seems that there is some bug with the configuration files of the bicouplers of the MK2 Expansion mod, the attached log is from a install with quite a few mods, but it's also happening with only freeIVA and the Mk2expansion mod loaded.

Full log attached :
KSP.log
i've read through it quickly, it might have something to do with the back nodes not being found ?

note: had previous version, but still happens with latest

Thanks for the report! Not really sure what happened there (maybe copy/pasted from the tricoupler) but it should be fixed.