false message about broken links

Example:
I get the message that the following link is broken:
https://www.razenberg.nl/betrekkelijk/

When I check it, by clicking it, I see the next address in my browser:
https://www.razenberg.nlbetrekkelijk/ (the slash after.nl is gone)

That sounds like a problem with relative URLs. How does this link look in the post editor (as in, what’s that actual HTML code)? What is the link URL, and what is the post permalink?

The permalink in the post editor is: https://www.razenberg.nl/betrekkelijk/

The internal link which I use in a post is:
betrekkelijk

Okay, but what is the permalink of the post where you used the “betrekkelijk” link? A relative link would be resolved based on the URL of the current post/page.

The internal link which I use in a post is:
betrekkelijk

Please surround the HTML code with ` (backticks) to display it as-is.

First the information in may mail:

Linktekst: betrekkelijk
Link URL : https://www.column-razenberg.nl/betrekkelijk/
Bron: betrekkelijk (2)

The permalink of ‘betrekkelijk (2)’ is:

https://www.razenberg.nl/betrekkelijk-2/

It looks like the redirection settings on https://www.column-razenberg.nl are messed up. For some reason, the site removes the leading slash from the page slug before redirecting to https://www.razenberg.nl. The result is that all links to pages (or subdirectories) end up redirecting to invalid domain names. Only the homepage (https://www.column-razenberg.nl/) redirects to the right domain name.

For example, all of the following links redirect incorrectly:

https://www.column-razenberg.nl/coaching/
https://www.column-razenberg.nl/liefde/
https://www.column-razenberg.nl/gratis-e-boek/

Looks like the plugin was reporting the links for a good reason.