without having changed any settings, and using the "info" button on the tv I can see the recalbox main menu displaying at 640x480, progressive, 60Hz, 4:3 aspect.
i can launch the game "Uranus" on GBA. this game came preinstalled. it displays at 1920x1080, 30hZ, 1:9 aspect.
but now i have tried several games on different systems and they all work so far. all displaying with the same settings as mentioned above.
the only thing i have done is change the tv setting "auto aspect". it was annoying that every time the tv changed video, it tried to adjust the aspect ratio. so i finally disabled that. i tried games with that tv setting on and off and it worked consistently.
the global.videomode=CEA 5HDMI
I really appreciate everyone trying to assist. but everything seems to be working fine and I have no idea why.
Ethernet seems to be the problem. I'm using the newest Version of GFE with a GeForce 1070. And if I use my Edimax W-Lan Stick on my RPI2 with Recalbox 4.1.0. Moonlight works flawless.
If I disable W-Lan and jackin Ethernet I have the same problem as Florian. So black screen with sound, sometimes I see also the Steam splash screen for some secons until it freezes or goes back to the Recalbox menu. So I'm happy that it works with W-Lan.