Anchor?

I can’t seem to reproduce that on my site. Anchor links are reported as working.

What status code and URL does the plugin show for that link?

The problem was then solved by itself.
After the article was public, it was no longer marked as invalid.
It was apparently only shown in the preview, while writing.

Ah, so the article was private before? In general, the plugin can’t check links to private or password-protected posts. When it checks a link, it attempts to load the linked page as a non-privileged visitor. So given a private post, it would probably get an “access denied” error when trying to retrieve it.