Network menu problem

Good morning. I have a confusing problem. One sub-site ( will not load from the network menu. Instead, the url redirects to my Defender masked '404' which is
I am unable to access either the front or back end from that menu BUT if I type in the full URL, I can access that sub-site. To futher confuse the issue, if I hover over the site name (in network) and select "New Post" it will take me to teh back end/new post!
David Wilks