Zoom for Salesforce - Invalid Access token.. Please contact your admin'

We recently set up Zoom for Salesforce using JWT app. It worked fine (for me, with my Zoom account). I recently tried to replicate this for my client and to enable the integration with their Zoom credentials.

Now, whenever we view a Zoom event in Salesforce, we receive an error to say ‘Invalid Access token… Please contact your admin’ and none of the Zoom details are visible. I have tried reentering the credentials for the JWT app and have followed your instructions here to the letter. It now no longer works even for my Zoom credentials/the original app.

I suspect something has changed - any ideas?