Cannot publish changes and creates a 0 byte file on the server

I have recommended Contribute to a customer to update their site and it's important they can do this. My version works perfectly but theirs does not.
They can log onto the site and select edit. They have control of the file if I try and logon so that seems fine. When they try and publish their changes after a long pause they get a message saying that the connection cannot be made. What's even more disturbing is that the file they are changing on the site is replaced with a 0 byte file therefore removing the page they are trying to modify.
Any thoughts on this please? I am stumped.