AMP Page URL is Error (5XX)
This means that the URL in question is an AMP Page URL, which returned Error (5XX).
Why is this important?
If an AMP Page URL returns a server error (5XX), search engines may be unable to crawl the page and access the content, which means it may not get indexed properly or appear in search results.
What does the Hint check?
This Hint will trigger for any AMP Page URL that returns a 5XX HTTP header response.
Examples that trigger this Hint
Consider the AMP URL: https://example.com/page-a/amp/
The Hint would trigger for this URL if it had a 500 (Internal Server Error) header response:
Similarly, URLs with other 'server error' statuses will trigger the Hint (i.e. any other 5XX response).
How do you resolve this issue?
AMP URLs need to be accessible in order to be indexed. In this case, the page was inaccessible due to a server error when Sitebulb attempted to crawl the page. This doesn't necessarily mean that this is always the case, and the page may respond ok for you now when you test it in the browser.
This is because 5XX errors indicate an issue with the server itself (rather than the specific URL, or even the website).
Server errors affect access to your site, for search engines and users alike, and you really want to get to the bottom of why they occur - you may need to deal with a developer or server admin to understand what is causing the server errors.
To rectify, you would need to ensure that the <link rel="amphtml"> tag is pointing to the correct (i.e. equivalent) AMP page, and ensure that this page is fully accessible to search engines and users alike, returning a 200 HTTP response.