Can't create menu using default WordPress settings

I've got this really weird problem here where I can create a menu using the default WordPress functionality, but it will not show in the header.

Here's my situation:
For a friend I've setup a server provided by Strato (https://www.strato.com/en_nl/) and we manage it with Plesk (12).

We moved one WordPress website and I installed another without any problem.

Now comes the weird part... whenever I want to add an item to the menu in the moved one, or add a new menu to the newly installed website nothing appears in the front-end. Same goes for using the default WordPress customize function under Appearance.

I can however create pages, posts, upload images and or adjust content, etc.

There are no errors when activating WordPress debug.

I've setup quiet a few WordPress websites but Ive never encountered anything like this.

Is there anyone who can point me in the right direction?

Cheers,
Leonard