Lemmy.zip has/had a mismatch between frontend and backend validation on usernames. When the frontend allows something that the backend rejects, you get some random "InvalidName" pop-up on form submission.

I tried several different variations of my preferred user name, but I couldn't figure out what was violating backend validation, so finally I settled on InvalidName. Which triggered a server error (500 error or timeout or something, I don't recall).

But finally, one glorious moment in time, the lemmy.zip server accepted InvalidName2 without choking, and the rest is history.