I have a 6-story BRB model that runs fine. As soon as I add a seventh story (diaphragm is different than stories below), I get an unstable structure detected. The node for which the instability is found occurs in the middle of the diaphragm. I have column ends fixed in all directions, beam ends released for torsion, and brace ends released in all directions. I've played with these fixities, but nothing seems to work. I also know it is not an issue with the different diaphragm because I used that diaphragm on the 6-story model, and that also runs fine. I'm pretty confident it is an issue with P-delta because the model runs when I turn off P-delta, but I can't figure out how to get it working with P-delta.
BRB can be excluded from the gravity analysis like tension-only braces. This can lead to an unstable structure for gravity loads. If you want to keep them for lateral loads only, then a secondary form of stability (i.e. a fixed column base or foundation spring) can resolve things. See: RAM Frame - Tension Only [FAQ]