Problem with adding a IPv4 static route

Hi there,

I'm running into a weird problem with adding a simple IPv4 static route.

My Zyxel is connected VPN site-to-site to another network B. In network B there is another network C with IP range 192.168.0.0/24.
Now I want to be able to reach 192.168.0.0/24 from my Zyxel.

I tried adding a static route with destination ip 192.168.0.0 subnet mask 255.255.255.0 gateway ip 192.168.1.2(vpn router on network B ).

I always get the following error message:
CLI Number: 0
Warning Number: 48000
Warning Message: 'Static route adding failed'

It still shows up on the table but doesn't work. When I try to ping it gets redirected to my WAN interface.
Any suggestions?