I tried signing up using the ctznry.vercel app to my own server (ctzn.leichtgewicht.at). When asked for the Display Name I entered it with a space "Martin Heidegger" and the following sign-up showed the confusing error message that my username contained invalid characters (space), returning to the previous screen it showed that it took my display name as user name. I changed it again and entered "martin" in both fields. Sign up worked. I was later able to change my name in the profile screen.
I tried signing up using the ctznry.vercel app to my own server (ctzn.leichtgewicht.at). When asked for the Display Name I entered it with a space "Martin Heidegger" and the following sign-up showed the confusing error message that my username contained invalid characters (space), returning to the previous screen it showed that it took my display name as user name. I changed it again and entered "martin" in both fields. Sign up worked. I was later able to change my name in the profile screen.