The namespace specified in the file ui/src/main/resources/SLING-INF/nodetypes/nodetypes.cnd of repository sling-org-apache-sling-app-cms is wrong.
Line 24 reads: <sling = 'http://www.sling.apache.org/sling/1.0'>
This should be: <sling = 'http://sling.apache.org/jcr/sling/1.0'>
This is a major issue when creating new repositories: when you build a custom starter package including Apache CMS and create a new repository with this starter package afterwards, the namespace of all nodes in the repository is automatically wrong. In normal operation, you won't notice that. But when you export content as package from a sling instance with the right namespace and import the content to a sling instance with the wrong namespace the import will fail. I did not test the other way round, but assume the issue also occurs there.
Suggested fix: change