[User 325245]

@"DaLanik"  great news ! Ambilight from the Windows HTPC is finally working !

First I changed a few small things in my router, which is better in general so thats fine.
Wasn't the solution though. Did keep an eye on some logging in the router.

Then I tried with Kodi on my phone. So its wireless. At first it didnt work fully, but when turning off hardware acceleration it did ! On both my wifi networks !

So, I figured out it must be something in the Windows HTPC, or in the network. Checked some things and finally checked internet options in Windows... and then I saw it... the ethernet connection was set to 'Public' profile and thus other devices in the private network cant reach it.
OMFG so stupid and embarrassing, but an easy solution ofcourse... set it to private and ofcourse ambilight works !!! 

Ambilight really is looking great !
Thanks for this addon, thanks for the help and suggestions.

 Had to be something with the network