I have heard many people bug me about this problem, and honestly, it's a pain in the ass because I don't know what is wrong with permissions.
It has done this to me every single time I buy a server with Multiplay.
There is nothing wrong with the way I've set up the permissions, it has something to do with Multiplay.
My evidence is the fact that I had the permissions running smooth and everything when I was running the server on my computer. But when I switched to Multiplay, everybody that wasn't an OP started getting these error messages. So, as far as I know, there isn't anything I can do about it. Sorry.