Rainloop Won't connect to user account

Question

Hello,
First of all, i am currently testing postfix and rainloop for mailserver. I know that there are some security issues when trying to run your own mail server. But i just want to do it for the fun and understands how it works.

I followed this tutorial to setup the server : (even if i am on ubuntu 18.04) https://www.tecmint.com/install-postfix-mail-server-with-webmail-in-debian/

My problem is the following :
During all the tutorial everything worked great. Sending mail and creating users with postfix is working. I can even connect with the admin account to modify the rainloop settings.

But whenever i try to connect using a mail account ([e-mail]), rainloop redirect me to the login page.

If i try putting a wrong password or an unknown domain, rainloop works as intended and gives me the "authentication error" or "domain is not allowed"

I tried checking the basic permissions or other tutorials about rainloop but it doesn't seems to correct my problem. Reinstalling it by following the official rainloop documentation didn't do much either.
https://www.rainloop.net/docs/installation/
https://www.rainloop.net/docs/permissions/

If you have any idea or any tips, feel free to answer me.
Thank you.