We have two identical sites (one is a staging site) hosted on different servers. Both are running the same WordPress version 4.8.5.
On one of these sites Vimeo videos are failing to load (previously these were working fine at some point in the past couple weeks). This has been tested using the default unmodified Seventeen theme to rule out any theme-related issues and the problem is the same.
For example, when I insert a Vimeo video link on the faulty site it just returns the URL for the video in the page eg:
Yet on the working site, with exactly the same page content the video URL above is automatically replaced with the Vimeo video player.
Screenshot from faulty site:
Screenshot from working site:
For testing we tried a Youtube video and this is displayed fine on the faulty site.
How can I troubleshoot this and work out what is causing the problem?