Network fails with 802.3ad after connecting second cable

I'm trying to setup Bonding in Lubuntu 20.04 LTS. I've got the onboard NIC plus a PCI card with two more NICs. All three ports should get connected to a Ubiquiti Switch US-8-60W, the three ports are already configured as Aggregate ports (which should support 802.3ad). My configuration in /etc/network/interfaces looks like this: auto lo iface lo inet loopback auto enp0s31f6 iface enp0s31f6 inet manual bond-master bond0 auto enp6s0 iface enp6s0 inet manual bond-master bond0 auto enp7s0 iface enp7s0 inet manual bond-master bond0 auto bond0 iface bond0 inet static address 192.168.1.11 gateway 192.168.1.1 netmask 255.255.255.0 dns-nameservers 192.168.1.1 bond-mode 4 bond-miimon 100 bond-xmit-hash-policy layer2+3 bond-slaves enp0s31f6 enp6s0 enp7s0 If only one cable is connected to enp0s31f6 (the onboard NIC), everything works correctly*. As soon as I connect a second cable, the network starts failing after half a minute or so. A bit hard to describe, sometimes I can't access the internet anymore but can still ping the router, sometimes pinging the router doesn't work either. In all cases, I can't reach the machine 192.168.1.11 from any other machine anymore. As soon as I disconnect the second port, everything goes back to normal. \* When I say "correctly", one thing is still odd when using one cable only. I've got a few virtual machines (Virtualbox) with static IPs and bridging mode. If I select bond0 as network adapter, I can't reach the virtual machines from outside (traffic from the VM works). When I switch the adapter to enp0s31f6, I can reach the VM again.