You may be having the same issue as I did: https://github.com/LemmyNet/lemmy-ui/issues/1652
I finally succeeded in submitting my registration application only after disabling the Bitwarden extension completely in my browser.
Do you use a password manager like Bitwarden, 1Password, LastPass, etc.?
I think this may be the same issue: https://github.com/LemmyNet/lemmy-ui/issues/1652
In my case, there was never any POST request/websocket communication (no network activity at all) after pressing the button.
I finally succeeded in submitting my registration application only after disabling the Bitwarden extension completely in my browser.