In some cases websites block the process we use to check if a page exists and this can result in false positives.
This means when we check the page the web server returns a code that says the page isn't there, but if you go to it directly the URL works.
You can double check the specific URL using a site like this one.
You can also re-test the links here.
We are considering adding a feature in the future that allows an author to mark such a link as fine (ignore it), but the current state is some may show as broken due to the information presented by the website.