I am running Debian GNU/Linux 11 (bullseye) and FreedomBox version 21.4.4
I was creating user accounts using email addresses.
Users trying to use either JSXC webclient or ChatSecure app, would have login fail.
Experimenting, I discovered that I could take an existing user, simply edit the user name via Users and Groups by removing @, and then that account was able to login.
While the user is logged in, I can change the account name and their ability to access JXSC is modified within the current session.
Log’s show no errors or warnings.
-SF