Read: machine files that parse
Check robots.txt, llms.txt and the sitemap on both hosts.
Before reading your pages, crawlers read a few small files: robots.txt, the sitemap it declares, and more and more often /llms.txt, a plain markdown summary of the site for language models. Each one has to answer 200 with the right content type and a body a machine can parse. A login page, a challenge page or your theme’s “not found” page served with status 200 is the most common failure: the status says the file exists and the body says it does not.
Check both hosts, example.com and www.example.com. A redirect on one and a different file on the other is a split nobody notices. Every URL in the sitemap should answer 200 and be the page’s own canonical address.
On your own site
In your report, open Read. Fix any file marked as served with the wrong type or with HTML in place of its content, and any sitemap URL that redirects.