Might help, but when I first tried signing up a few months back I had the same issue, it was bc the username I wanted was too many characters and a db error was just getting swallowed and no message came back telling me it's too long.

The reason I even know this is bc I'm slightly insane and decided to spin up my own lemmy instance just so I could debug it. I never submitted an issue on GitHub for it, because on top of being insane, I'm also incredibly lazy