back office not working after Design - Theme & logo - Choose layouts

[sygram]

Hi there,

i installed and configured latest prestashop 1.7. Everything was fine till i clicked Choose layouts (Design - Theme & logo). After that the backoffice just gave me a blank screen. I managed to change the file defines.inc.php and set debug mode _PS_MODE_DEV_ to true. Somehow my shop's backend works now, but no error is displayed. So i am forced to use debug mode always now because when i set it to false, i always get the blank screen. Front works fine in both cases.

Does anyone know how to fix this ? The only thing that i did is Choose layouts.

Thank you very much. I really appreciate your replies.

Regards