WAC doesn’t work from specific SharePoint Web Applications

Problem

WAC (Office Web Apps) works from some SharePoint 2013 web applications of a farm, but not from some. You get the normal errors when trying to view document on the problematic web application via WAC, like:

Sorry, there was a problem and we can't open this document. If this happens again, try opening the document in Microsoft Word.

Solution

Make sure the AAM settings for the web application are configured so that the URL being used is set as Default Zone. There are of course millions of other reasons for the error above, but in my case everything else was configured correctly.

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