Hi Andrew, thanks for looking into it.
I’ve just spent all morning, trying it out on a fresh install, and a clone of my client site where I had the problem.
Everything was inconsistent to start with – creating new posts allowed me to embed the video normally, but trying to add videos to existing posts (that existed pre WP5) was hit and miss. I tried taking all of the content from the old post, including the featured image and tax settings and applying them to my new post, but that still allowed the embed to generate on the new post.
Cloning the old post, and trying to embed in that had no luck either, so I started to think it was something in post meta or transients of old posts.
On my cloned site, I installed https://www.ads-software.com/plugins/rvg-optimize-database/, and in the settings deleted transients and selected ‘Clear oEmbed cache’. Ran the optimisation. This time my old posts on my site clone (separate server) allowed the embed to generate. “That’s great” I thought, I’ve cracked it. Unfortunately when I tried this on my original site on my local machine, installed the same plugin and optimised, my local old post still doesn’t let me generate the embed.
When the embed doesn’t generate by the way, I’m seeing no errors in Chrome console or debug logging.
I literally give up. I’ll update the clients site and recreate the new posts with vimeo in them if I have to. I just wanted to report back in case you hear of anything similar.
If you’d like a copy of my local install for testing (there’s no personal data or anything confidential in it), feel free to email me – check my profile for our domain, and it’s chris@ – I can dropbox you a copy.
Thanks again