Simple - during registration Flarum wants to send email with invitation message. If you don't have email driver configured, then you have error message, but new user is already created, so you can log in despite error.
The simplest solution is to use log mail driver in local environment, so that this emails are written to log file. In production you have to configure smtp or some other driver, such as mailgun.
So it is not a bug, it's a feature😉
