Add Extension - Server Error

New install, during install I was able to add a couple of extensions; eg Paypal which is working but if I try to install any additional extensions from the store I get:

Argument 1 passed to Shopware\Core\Framework\Store\Struct\CartPositionCollection::__construct() must be iterable, null given, called in /homepages/43/d416919653/htdocs/ on line 44

Really not sure what to do about it. Other than jumping in to the code is there any configuration I can check first?