IMAP Dovecot cannot connect

I am trying to set up my own mail server (for 'fun'). I followed this tutorial to the bone. But I get this error message:

imap-login: Disconnected (auth failed, 2 attempts in 9 secs): user=<dayuloli@mydomain.com>, method=PLAIN, rip=203.210.7.43, lip=1.2.3.4, TLS: Disconnected, session=<VDW4ua8NzADL0gcr>

I am using the SSL certificate and key I have for my nginx web server, on the same machine.

I disabled plaintext authentication by adding these two lines (as per tutorial) to /etc/dovecot/conf.d/10-auth.conf:

disable_plaintext_auth = yes
auth_mechanisms = plain login

But the method in the log entry still says PLAIN, and TLS is Disconnected. Could that be an issue?

There're too many configurations to post them all here. if you think you need more information, please tell me which parts and I will update my question.