Open up https://chat.mozilla.org in your browser. Although there are multiple apps, you don't need one.
Cannot reach homeserver Ensure you have a stable internet connection, or get in touch with the server admin
https://chat.mozilla.org/#/register
It's trying to make a GET request to this URL and 404'ing:
https://chat.mozilla.org/config.chat.mozilla.org.json?cacheb...
That brought me to this page:
https://mozilla.modular.im/_matrix/saml2/authn_response
Which only displayed the word
ForbiddenThey mean it when they say "User ID can only contain characters a-z, 0-9, or '=_-./".
Is that really so esoteric?
I thought you said it was web-based?