e newsletter not working with theme

Aghhhhhhhh Heeeeeeellllp

Hi all

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.

Kind regards

Stef :slight_frown: