Internet on Android apps fail when on ethernet cable

I have a strange problem. When connected to WiFi, I have no issues. When I connect a UTP cable to my device via a USB ethernet adaptor, I can browse the internet, but apps won't work. I can, for example, browse https://www.youtube.com , but the YouTube app fails to connect to the network. I tested on 2 medical smartphones (Spectralink and Ascom Myco 3) on 2 different networks. No firewalls or spam filters on the network. Both have the same issue. I have a medical app that can not connect to its server on port 443 when on the cable. When using a web browser to connect, it says it doesn't trust the certificate. Again, on WiFi, it's fine. It seems to me that the cabled network has a different way of handling certificates. Can there be a difference? Any suggestions are highly appreciated.