Well the "not found" error may indicate a server issue. It sounds like the server is interfering with the request if it says the resource was not found. It could be due to a misconfigured Apache module like mod_security. You would have to ask your host about this. Ideally there will be logs on the server that will reveal the nature of the problem and point to the cause. Or more generally I would ask your host to disable mod_security and any other restrictive Apache modules to see if that fixes it.