After having a second thought I disabled the net balancer and added static routes for remote gateways instead. This fixed the problem.
Then also added a default gateway pointing to one of the router and work with this in place to.
I don’t know if this is the intended zeroshell config for handling this, probably more a workaround, because if net balancer is disabled the internet traffic from the clients on the LAN behind zeroshell will have to go always one route.
Anyway this will work fine in my setup.
Thank you ppalias, you helped me to get this going. You are Great!