Adding two GREEN zones breaks connectivity

NethServer Version: 7.9.2009
I have a working Nethserver installation with 3 interfaces: 1 GREEN (eth0) and two RED in MultiWAN (eth1 and eth2). All the internal networks, including servers accessible from the outside, are connected to the GREEN interface via a router and in nethserver there are routes configured to reach those networks. Squid is configured in transparent SSL mode for the GREEN zone and the only additional application enabled is Threat shield.
Recently I added a fourth interface (eth3) to Nethserver with the objective to gradually move servers to a separate zone. I’m using a different network address which doesn’t overlap with the existing ones. At first, I configured this new interface with and ORANGE role, moving just one server to this network and everything worked fine.
However, I will need that servers in this zone go trough squid and this feature is not supported i Nethserver, so I decided to change the role of eth3 to GREEN and immediately all clients in the first GREEN zone (eth0) lost connection to Internet. I also tried changing eth3 to BLUE role with the same result. Fortunately I didn’t lose connection to Nethserver and restoring eth3 to ORANGE solved the problem.
Do you have any hints on how to locate the source of this problem?

Are in place any rules on the firewall?
Custom routes?

Hi Michael.
Yes, there are rules on the firewall and custom rules for the networks behind the router connected to the first Green interface (eth0).
However this shouldn’t matter, as the role I’m changing is for the new interface (eth3), which is on a different network that doesn’t overlap with the rest and neither has firewall rules or routes defined for it.