Emails are not being fetched for one specific mailbox. Other mailboxes work fine and email fetching is enabled globally.

Go to Admin Panel > Emails, open the failing email account, go to its Remote Mailbox tab, verify the settings, and click Save Changes with the Remote Mailbox Status set to Enabled -- this triggers an authentication test that will show if the credentials are failing. Also: if you have a cron job running, disable the 'Auto-cron' checkbox for email fetching to prevent it from conflicting with your cron job and causing duplicate tickets.