This seems to be a bug caused by a bug. If that makes any sense.

A war lead me in to the home system of another family in Supernova.

So a war between fam A ( me ) and fam B.
Leads to planets in the homesystem of fam C.
Fam C is empty family.

I was able to take planets from Fam B in there. But now all of the sudden it says I can't attack in that system.
I was able to attack fam B on other systems. Just not in there.

I have no idea how I can contact a MOD to give further details ( systems, more names etc ).