@fisi I think @nebulon responded to your ticket from support. If this is the same support ticket, the reason is that your server has 2 NICs and connected to the internal and external network. This is not a problem but the routes are set up incorrectly in ubuntu. The default route is going via the internal gateway. I think you have to adjust the netplan to fix the routing situation .
(I hope I am replying about the correct tickte 🙂 ! )