A client uses X theme 9.0.5 with Cornerstone 6.0.5 and Yoast SEO Premium 14.2. All other plugins are up to date.
They recently reported that they weren’t able to save changes to Cornerstone pages.
Unfortunately I wasn’t able to see anything obviously flagged in the server logs.
I was able to narrow the problem down to Yoast - if Yoast is deactivated, Cornerstone saves just fine. If Yoast is activated Cornerstone gives the “Uh oh! Failed to save” error.
Let me know when you get it fixed, or if you need access to see for yourself.