I’ve received the same error, and I was able to come up with a bizarre workaround for it, but I don’t believe that I’ve necessarily solved the problem. I found that changing the URL on the pages that I saw the error on bypassed the error and sent the email. e.g if https://www.xyz.com/4097-2/ generated an error (4097-2 is a specific URL that failed on the site I used to generate a failure), I’d change the URL to https://www.xyz.com/completely-new-page/ and all was well. We do have a caching plugin as well (WP Super Cache) but I disabled it, so I know that’s not the issue either.
Wondering what the underlying cause may be.
