Anyone running rebirth with BTB's game changes needs to ensure the BTB loads after all the plugins rebirth adds, MLOX will not detect this conflict at this time and you will need to put in a user defined option. Easiest is to use the NearEnd lsiting.

So, what's the actual problem between these mods? Is it a http://www.gamesas.com/topic/1397841-relz-boats/page__view__findpost__p__21521014 again?

I also thought there was only one plugin in Rebirth: Morrowind Rebirth .esp

Probably that both Rebirth and BTB changes things like game settings, values etc.

Yes Dragon is right, there's only one plugin in Rebirth.