Products not available in manual backend orders

Can not add product to manual order
I am trying to add an order in Admin-Sales-Orders-Insert
I am unable to add a product to the order.
When I type the first few letters of the product I get the drop down list ok - then I get the options list but if I then press 'Add Product' I get a warning pop-up 'The page at https://beds.slumberz.co.uk says: error'
I added an SSL certificate to my site yesterday and this is my first attempt to add an order manually so I am wondering if the two are linked.
I have updated the SSL settings Admin-System-Settings-Server and amended the config.php and admin/config.php files to reflect the https
Everything seems to be working OK apart from this problem
I am running Opencart 1.5.2.1