This is the line that causes trouble:
private List<Ping> serverPingList = new List<Ping>();
And this is what the Log yells at me:
CS0246: The type or namespace name List1’ could not be found. Are you missing a using directive or an assembly reference.
So after that was added, I forgot to add that. I get even more errors, well thanks - that solves everything
– DryTear