Importing hierarchy content for hierarchies with categories that have a relationship attribute set not supported

I have never received this error before. Why am I getting this error and how can I fix it?

Symptoms

Importing hierarchy contents for hierarchies that have categories with data that is specified for relationship attributes is not supported.

Resolving the problem

Important: This workaround results in a loss of relationship data.
  1. Extract the compressed file that was generated from the company export.
  2. Browse to the HIERARCHY_CONTENTS directory.
  3. Edit each CSV file and delete the column or columns that specify relationship data. Remember to save the files after editing them.
  4. Compress the files again. The directory structure must be the same, where the file ImportEnvControl.xml and the archives directory are at the root level.
  5. Import the updated compressed file.