Not able to log in to Webmail/IMAP on Plesk for Linux server: error: Permission denied

Symptoms

Unable to log in to Webmail/IMAP for example.com. It fails with the below errors:

CONFIG_TEXT: courier-imapd: info@example.com: chdir(/var/qmail/mailnames/example.com/info) failed!!
courier-imapd: error: Permission denied
courier-imapd: LOGIN FAILED, method=PLAIN, ip=[::1]
courier-imapd: authentication error: Permission denied

or

CONFIG_TEXT: dovecot: service=imap, user=info@example.com, ip=[127.0.0.1]. Error: opendir(/var/qmail/mailnames/example.com/applications/Maildir) failed: Permission denied (euid=30(popuser) egid=31(popuser) missing +r perm: /var/qmail/mailnames/example.com/applications/Maildir, we're not in group 516)

Cause

Incorrect permissions for the /var/qmail/mailnames/example.com/ directory.


Resolution

To apply the solution from this article it is required to have SSH access to the server. Please, contact your hosting provider support or server's administrator if you don't have SSH access or ask them to apply the solution.

First of all, make sure that the error is the same as in the title.

Connect to the server via SSH and correct permissions that should be as follows:

# stat /var/qmail/mailnames/example.com/ | grep Access
Access: (0750/drwxr-x---) Uid: ( 30/ popuser) Gid: ( 31/ popuser)
Access: 2015-12-23 16:59:20.527983735 +0700

Make sure that the /var/qmail/mailnames/domain.tld/applications/Maildir/ has the same permissions.