During Office 365 CSP migation procedure importsubsCSP.py script fails with:
HTTP Error 500: Internal Server Error(The CSP customer xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx does not exist.)
The cause is a software-related issue APSA-18978.
The issue has been fixed in the Office 365 package version 17.1.1. Please contact your Technical Account Manager or Pooled Team Associates(email@example.com) to schedule an Office 365 update.
As a temporary workaround the regular importSubsCSP.py should be replaced with the new one from Office 356 package version 17.1.1.
Also, the fixed script can be downloaded here.