Today (or yesterday) RTL decided that you should no longer use the RTLxl addon for Kodi and at the root level displays that you should use the RTLxl website instead. Thank you RTL, but I want to watch on my television without buying something new. (And I wouldn't mind if you put the commercials in, because I guess that's the only reason why you want people to use the website or app.)
Now what could RTL have done to block the Kodi addon? Actually, not much, since Retrospect (which unfortunately mentions on its website that it's no longer available for legal reasons) was still working. So my best guess was that they were applying a user agent check and since the RTLxl addon is kinda old that would certainly be easy to do.
So I went into the RTLxl addon folder in Kodi (~/.kodi/addons/plugin/video.rtlxl/) and started editing addon.py. There was one outdated User-Agent there, so I changed that to something more modern (thank you Google). Restarted Kodi, no luck. But addon.py is relatively small, so I went into resources/lib and opened rtlxl.py, which contained another few User-Agent strings. Replaced them all, restarted Kodi and my wife's happy again!