Have they ever stated the limit of the BOS frags slots? I think 70 is the limit for BOS souls/keys. If they removed the keys from the BOS souls inventory that would open up a fair amount of slots.
They definetly keep adding keys with no relief in sight for increased slots.
I came across a similar overflow issue between soul slots and frag slots. Luckily I had toon slots available. I went in circles until I tried to reinstall the game. I think there are several variable that can cause the overflow issues.