Everything was fine, newsletters sending etc, however, after a colleague created a new newsletter on another Mac, I have attempted to edit it this morning and BOOOM.
I now seem to be stuck and the site is broken.
When pressing edit soon a newsletter I get
The site then reverts to the default twenty fifteen.
The exact same happens with twenty fifteen activated.
I have tried deactivating all plugins etc but the same happens.
What is more confusing is that pretty well exactly the same set up is in use on a subdomain and this is working fine.
Please help this is our most important live site.