obviously the same command from shell works fine...
i used ubuntu 16.04 from proxmox default repository
can you post your /network/interfaces file?
i have two virtual interfaces...
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet static
address 192.168.18.180
netmask 255.255.240.0
gateway 192.168.16.1
up route add -net 10.110.0.0/16 gw 192.168.18.200
auto eth1
iface eth1 inet static
address 10.56.153.131
netmask 255.255.255.0
auto eth2
iface eth2 inet static
address 10.56.153.132
netmask 255.255.255.0
auto eth3
iface eth3 inet static
address 10.56.153.133
netmask 255.255.255.0
auto eth4
iface eth4 inet static
address 10.56.153.134
netmask 255.255.255.0