When you modify something in a .zip folder, you must use setModPaths(getModPaths()); or discoverfile("add-ons/FileZip.zip"); before exec("add-ons/FileZip/server.cs"); (or client.cs or whatever) because the zip file isn't updated while making a folder with a description.txt and a server.cs is always updated, I believe. I have had this problem many times until I learned it was this that was causing it.