Ok, seems like it’s MTU issue once again:
Try to run these commands in ivpn-proxy:
sudo ip link set dev eth0 mtu 1420
sudo ip link set dev <your ivpn wireguard interface> mtu 1340
And this command in appvm:
sudo ip link set dev eth0 mtu 1340
And check if other sites in appvms firefox will start to work.
UPD:
I’ve changed MTU from 1360 to 1340.