Youtube not playing in Wifi

I have this real strange problem that youtube is not playing on my Onda Tablet (dark screen and "problem while playing"), when I am in my home wifi. When I use my mobile as a hotspot and connect my tablet to it (also via wifi) it's working. I use several android devices and the Onda Tablet is the only one, which does not play Youtube. Other Video Players work without problems. This also happens, when I browse to m.youtube.com in a browser on the tablet. Here's what I already tried: - Clean Cache and data of youtube app - reinstall youtube app - manually downgrade youtube app - changed DNS of the tablet to 8.8.8.8 - Factory reset the tablet - Reset my router My only guess is a misconfiguration in the wifi options of the tablet but I could not find one. My Tablet: Onda air v989 running Android 4.4.4 Any help would be much appreciated. Thank you!