Thank you for your answer, and yes, you are completely right, when you are saying "It sounds like when you installed UNA the trailing slash was removed from the Site URL."

So I reviewed my installation-step-screenshots and I found the culprit, which is me.

I changed the "Site URL" under "Site configuration" > "Site paths" during the installation and forgot to add the slash "/" at the end of the URL.

So I ask everyone's forgiveness for worrying about such a stupid rookie mistake.

From a developer's point of view, however, one could also learn from this and possibly ensure programmatically that the presence of this backslash is ensured before the user clicks the "Submit" button or that at least some error message is made that the slash "/" is missing in the "Site URL" field.

Anyway, many thanks to everyone helping me out with this issue.