Every single username I try gets marked as "prohibited word or character"

Tried to change my username to what I use on other services, back2bizniz, but it got flagged as prohibited. Started trying different variations and stuff, some of them worked, some didn't. But now literally everything I type gets marked, even if it was okay when I tried it before. It's just not letting me change my username anymore. How do I fix this?

(Also side note: Does anyone know WHY these names would be banned? I tried swapping out the number just to see what would happen and specifically 2, 4, 7, and 8 were banned, but all other numbers were fine. What's up with that?)