Re: Unknown remote IP negotiations error
Well, it could be someone trying to brute force their way into your network. If you only have site-to-site tunnels configured then you really don't have anything to worry about. If you have client IPSec configured then there is a very small chance they could eventually get in (very small).
If it's definitely always the same remote subnet and you have client IPSec configured, you could always put a block rule for that subnet above your IPSec policies. Or, put a blackhole route for that subnet in your static route table. Or, if you have a router outside your firewall that you have control of, put the blackhole route there.