When you create a Minefold server, you essentially create a world locally then upload it - I think Minefold then treats things like a mix between single and multiplayer. I think that right in the spawn area you can't create or destroy bricks because its 'my' spawn and I created the single player world and then uploaded it.
I didn't discover this until after I sort of started building stuff and Enron popped in to mess about. Outside of RIGHT in the spawn area, you should be good to go.