Configuration Management is broken between two site environments, I have no idea how to fix it
Aaaaaargh! After banging my head against the wall I found the issue. I checked the yml output (either single export tab; the /sync folder; or in the compressed export file) and found the 'broken' line:

dependencies:
  config:
    - core.entity_view_mode.node.teaser
    - node.type.leistungen
  module:
    - null
    - node
    - user


I removed the '- null' entry and retried the import - it worked fine.

MegaDoh!