The Combat Tag kills whoever is in each world. So say I die in dungeon and I respawn in the donor spawn. It says I'm tagged for 10 seconds and cannot log out. I do /back, and it says I logged out, so it kills me.
I think this is the real issue here ^^^ Since we have a lot of worlds on this server, it kills you in the world you died in if you do /back (as a donor) to go to that world.