Coupon theme extras doesn't appear

Hi,

I just installed the last version of the coupon theme (8.1). When I try to add a coupon or try to edit an existing one, the ‘coupon theme extras’ doesn’t appear on the page. This happens on my hosting. Please have a look at the attached screenshots. I also switched on the debug mode and got an extra screenshot.

My site is installed on a subdomain (like vouchers.mydomain.com). I don’t have any active plugins. I have activated everything on the PHP check list.

I don’t have this problem with the CP installation on my macbook.

Many thanks,
H