@dotdash : ok, thanks for your comments, I am running it now that way (with the other UDP port getting a 10.6.x.y) gateway and it seems it's working without any flaws. It can only be a /16 mask from the provider because the problems occured with GW1 having 10.4.A.B and GW2 having 10.4.C.D so it has to be a /16 mask otherwise with a /24 mask there wouldn't have been these conflicts.
@cmb : No I didn't add the gateways for the VPN clients on my own, but I somehow "renamed" the automatic ones (through adding a gateway based on the automatic created gateway (+ button on the gateway), entered an alternative monitor IP and shortened the name, then saving) -> they then had a shorter name and the "automatic" ones disappeared automatically. But I don't see a mistake in doing it like this because I did that from the beginning on and it's working without any flaws. I did the copying workaround because you can't rename the automatic created ones.
So I removed the BUG from headline, thanks!