Cannot create document in the GoGreen Enterprise using create content buttons.

Scenario:

1. Go to any GoGreen channel and try to create a banner document using manage content "+" button.
2. Click save on step 2. 

Problem: the toast message says: "Unable to save the documents contains invalid content"

There is a warning in the tomcat console:  [org.onehippo.repository.jaxrs.api.ManagedUserSessionInvoker.invoke():129] User session should not have changes at the end of the request. Refreshing them now

Same happens with other documetn types.