Thanks for your time and effort! Appreciate it.
Sorry, I was a bit sparse with my Infos. I’ve managed now to (dirty) upgrade from 3.0.4 (a14) to 3.0.4 (a15). Fairphone 3 community.
This Hint made the difference for me. 
After altering the URL and erasing the “IMG” part, i was able to perform sideloading without any hiccups. I used this thread as guidance. No need top update the recovery.
Nice to have such a helpfull community.