First steps in Dovecot; IMAP not working

[Moi]

Hello,

I've made some more tests and I still can't receive mails; sending them
still works. I don't receive any error message, just the mails that are
supposed to be received won't come.
In the mail logs, I find only this relevant line:
dovecot: imap-login: Aborted login (no auth attempts in 2 secs): user=<>

This line (which I shortened to remove IP addresses) seems to indicate
there's no user referenced, although I've set the field in Outlook. Is this
a problem that looks familiar?

I'm sort of clueless without having an error message.