Mastodon does not support login with usernames containing a dot

[nebulon]

This is just a thread to document what we have found debugging a user's instance.
If the login, using a Cloudron account, results in a blank page then one reason for that could be, if the username contains a dot. The app logs would show ActiveRecord::RecordInvalid (Validation failed: Account username only letters, numbers and underscores)

Currently there is no fix upstream and it is unclear if it will be supported given in the future. So the only solution is to not use a dot in the username.