I solved it finally. As I thought one way of fixing it was to target the router. I was one google search away from a good description of the issue. Serverfault.com : Loopback to forwarded Public IP address from local network - Hairpin NAT
Luckily my dlink router could be fed with a new firmware from dd-wrt.com and after defining my port forward rules everything just works (so far). Apparently the so called NAT loopback functionality is enabled by default. And as a bonus the router firmware seems quite capable!
Thanks @steve1 and @DieterL for your thoughts and suggestions. Hopefully this thread can be of use for someone else as well.