Thanks for your help but I can't update my cluster, if something breaks I don't have time to fix it at the moment and I need it up and running.
In case is useful for you to fix a possible bug I can run some tests if you want.
The problem might be related to a particular container as the command doesn't fail in other containers. Also the command does return an error but the interface is created:
root@pve2:~# pct set 126 -net3 "name=eth3,bridge=vmbr0,ip=192.168.5.126/24,gw=192.168.5.100,tag=5"
root@pve2:~# pct set 126 -net3 "name=eth3,bridge=vmbr0,ip=192.168.5.126/24,gw=192.168.5.100,tag=5"
root@pve2:~# pct config 124
arch: amd64
cpulimit: 1
cpuunits: 1024
hostname: test3
memory: 512
net0: bridge=vmbr0,gw=192.168.1.1,hwaddr=32:33:36:33:63:65,ip=192.168.1.124/32,name=eth0,type=veth
net1: bridge=vmbr0,gw=192.168.5.100,hwaddr=3A:39:36:30:64:32,ip=192.168.5.124/24,name=eth1,tag=5,type=veth
net2: bridge=vmbr0,gw=192.168.6.100,hwaddr=36:34:31:36:37:34,ip=192.168.6.124/24,name=eth2,tag=6,type=veth
ostype: ubuntu
rootfs: nfs-ssd:124/vm-124-disk-2.raw,size=4G
swap: 512
root@pve2:~# pct set 124 -net3 "name=eth3,bridge=vmbr0,ip=192.168.5.126/24,gw=192.168.5.100,tag=5"
Use of uninitialized value in pattern match (m//) at /usr/share/perl5/PVE/LXC/Setup/Debian.pm line 217, <GEN64> line 7.
Use of uninitialized value in pattern match (m//) at /usr/share/perl5/PVE/LXC/Setup/Debian.pm line 217, <GEN64> line 23.
root@pve2:~# pct config 124
arch: amd64
cpulimit: 1
cpuunits: 1024
hostname: test3
memory: 512
net0: bridge=vmbr0,gw=192.168.1.1,hwaddr=32:33:36:33:63:65,ip=192.168.1.124/32,name=eth0,type=veth
net1: bridge=vmbr0,gw=192.168.5.100,hwaddr=3A:39:36:30:64:32,ip=192.168.5.124/24,name=eth1,tag=5,type=veth
net2: bridge=vmbr0,gw=192.168.6.100,hwaddr=36:34:31:36:37:34,ip=192.168.6.124/24,name=eth2,tag=6,type=veth
net3: bridge=vmbr0,gw=192.168.5.100,hwaddr=3A:33:63:36:35:39,ip=192.168.5.126/24,name=eth3,tag=5,type=veth
ostype: ubuntu
rootfs: nfs-ssd:124/vm-124-disk-2.raw,size=4G
swap: 512
root@pve2:~# md5sum /usr/share/perl5/PVE/LXC/Setup/Debian.pm
4fb86ef1633ba27867d98680387d28eb /usr/share/perl5/PVE/LXC/Setup/Debian.pm