Default Root is not valid or does not exist.

If you receive a error message while subscribing or space configurating that looks like this:

Ups, there where some problems: - Default Root is not valid or does not exist. Please define a default content type that is configured with "Act as content type"'

Navigate to your Space Settings and the General Settings. There you can find the "Default Root" option which is not set for you space. Select any of your content types to be a default root so you can continue with your space configuration or subscription.