Hi Vikas, thanks for your answer!
I'll check the routes this afternoon, but I can access the WebUI when the trust interface is on an untagged port on the switch, but while doing this I can't reach the router anymore, as he runs on a tagged interface.
I also tried adding a new zone on the firewall attached to eth3 with management enabled and connected to an untagged port on the switch.
By using this way, and with only eth3 connected, I can access the WebUI with no problem as long as eth1 is disconnected, and at the opposite, when eth1 is connected and eth3 disconnected I can reach the router without any problem, but both connected won't work because it will create a loop and indefinitely repeat broadcasts...