The problem is only in the Test-Newsletter. Strange enough because I send the Test to test the message … Anyway in the sent newsletter the URL works and to me it looks like some kind of tracking.
This fact leads to my next question: I use the newsletter plugin because it is conform with General Data Protection Regulation by saving all data in the WordPress-database. Is it still conform with General Data Protection Regulation that the click on links in my newsletter are tracked? And who gets information about tracking?
Thanks for answer, hopefully before 25 may …
Problem solved, thanks to Stefano’s spot on analysis.
In my case, when I inserted https://<my-domain>/path links on my newsletter, and sent a test email, it was converted to http://<my-domain>/<tracking-code> inside the email.
Turned out that on my site’s Global Configuration, the site address was still with http://. Turning that to https:// and the test email now comes with https://<my-domain>/<tracking-code> and the link works. No more “URL not verified” error.
So, the test newsletter used the configured site address to compose the link on the email message. The webserver is configured to rewrite all http requests to https, so it messed all up.