yeoric

As I said, all is well if I switch off the Secret Content plugin. Without that, I get a list that includes basically ALL links in the site shown as 404. A few examples:
https://wp.flyhi.de/alle-personen/ (to a page)
https://wp.flyhi.de/balin-ein-zwerg-von-durins-volk/ (to a post)
https://wp.flyhi.de/alle-personen/#e (to an anchor inside a post)
I created a link list with a different plugin, and I confirmed that every site-internal link is affected.

It seems obvious that the Broken Link Checker does not act from within the context of my browser session, because 404 errors are exactly what you get with the Secret Content plugin if you are not logged in. You should be able to verify this if you install the Secret Content plugin on a test site (and mark the pages and posts that contain links as “Show this to logged-in visitors only”).

So I have a workaround: For the link check, I just switch of the Secret Content plugin for a few minutes, then I switch it on again.
IF the login requirement is the reason for the observed behaviour, maybe the Broken Link Checker could store a username and password to create a logged-in environment for its test runs.

I think we can close this case. I am willing to do more tests if you require them for further checks, should the behaviour of your plugin be different from your expectations. As I said, I changed my rating to 5 stars.

I can confirm sabinevi’s post that the Strato Guestbook Spam Filter seems to be the culprit. It is under the security settings. Once I switched the filter to “not active” everything is fine.

1 Star reduction for the “small gripe” mentioned, of course. I see no option to center the slider instance vertically, this means it pops to the upper viewport margin, this means the sticky header hides part of it. Please correct me if I overlooked an option.