So, the first thing I tried was to set the default language back to en_US.
After that, node repair worked without a hitch.
Then I changed default language to ‘de’ again, followed by
> ./flow node:migrate 20140326143834
Comments
Migrate from no node dimensions to default dimension values.
Successfully applied migration.
> ./flow node:repair
Generate missing URI path segments
Could not find "/sites" root node
Run several operations to ensure the node integrity
Checking for nodes with abstract or undefined node types ...
Checking for orphan nodes ...
Checking for disallowed child nodes ...
Checking for undefined properties ...
Checking for missing child nodes ...
Checking for child nodes that needs reordering ...
Checking for missing default values ...
Node repair finished.
>./flow configuration:validate
Validating all configuration
All Valid!
doesn’t seem to have any consequences, though.