Hyper-v external virtual switch only gives connection to 1 vm

We recently got a new Supermicro Server (X10DRi-Ln4+ MB, Intel i350 Network cards) and have problems with Hyper-V:

The Server is running Windows Server 2016 Datacenter with the latest Updates und Drivers installed. The Hyper-V Role is installed. The problem we are having is with the connectivity of the VMs being hosted by the Server.

When I setup a virtual switch for external usage and give it to a VM (VM A) everything works as expected (as in VM A gets the correct IP Address and is accessible through that Address). Now when I try to connect the same Virtual switch to another VM (VM B) the VM gets no connection at all. Instead, the connection within the VM shows up as unidentified connection. When I turn off VM A, VM B suddenly gets a connection and receives an IP Address. We tried both DHCP and manual IP configuration with the same result. The VMs we are using for testing purposes have different MAC addresses.

This behavior is the same for all NICs: I can connect exactly one VM to a virtual switch, if I connect any further VMs to the switch I do not get any connection on the additionally added VM. All VMs are running Windows Server 2016. We tried this with existing VMs and newly created VMs both with the same result.
We are already running two hyper-v hosts in the network with multiple vms per external virtual switch (difference: hpe server with Broadcom nics).

So far, we tried the following:

• Enabling/Disabling virtual machine queue
• Reinstalling the OS on the physical server
• Trying different Drivers and setting inside the driver for the physical network ports

We are pretty much out of ideas. So if anyone of you had similar problems we would be happy to get pointers on how to fix the problem.