Webmail or mail client does not show any emails

Symptoms

When logging in to webmail or a mail client, the email list does not show up. Instead, one of the following error messages appears:

Server Error: STATUS: Internal error occurred. Refer to server log for more information.

And the following error messages appears in /var/log/maillog:

dovecot: imap(user@domain.tld)<28033>: Error: Mailbox INBOX: Index is read-only, can't write-lock /home/username/mail/domain.tld/user/dovecot.index.log

Description 

This issue is caused because of incorrect file permissions and ownership.

For example the following:

# ls -lah /home/username/mail/domain.tld/user/dovecot.index.log
-r--r--r-- 1 username username 944 May  5 14:32 /home/username/mail/domain.tld/user/dovecot.index.log

While it should be the following:

# ls -lah /home/username/mail/domain.tld/user/dovecot.index.log
-rw-r----- 1 username username 944 May 5 14:32 /home/username/mail/domain.tld/user/dovecot.index.log

Workaround

You can solve this issue by setting the correct permissions: How do I change permissions on a file or folder?

If all of your domains and email accounts are affected, it might be more efficient to restore the files from a backup: chown or chmod was recursively ran on the wrong directory, is there a way to fix this?