To resolve this language and region mismatch, follow these steps:

Quit out of QuarkXPress Server.
Go to Apple Menu > System Preferences and click on International.
Click on the Formats tab.
Change the Region to 'United States.'
Restart QuarkXPress Server.

Background:
In order to check articles in via WebHub, the locale settings and Format settings on the computer running QuarkXPress Server must be the same language and region. In the case related to this solution, the locale settings in the ServerApp.properties file were set to 'en', but the Format settings were set for German (International > Formats > German)