NSX Manager upgrade fails during Data Migration steps with failure message "Unexpected error while upgrading upgrade unit: Data migration jar is not running"

Products

• VMware NSX

Issue/Introduction

• Found "Data migration jar is not running" on the /var/log/upgrade-coordinator/upgrade-coordinator.logs on the NSX manager nodes
• Couldn't see logical-migration log files on the NSX manager nodes 

Environment

NSX-T 4.1.0.2

Cause

• MP fabric module was updated multiple times via API. These updates inadvertently removed the 'from-version' and 'to-version' MUB repo-list entries, which consequently caused the deletion of the corresponding directories from the other two nodes. 
• repo-sync was incorrectly marked as successful across all three nodes.

Resolution

• Restore the NSX-T manager with the backup taken before upgrade process and continue the upgrade process again
• Collect the output of the same API (GET https://<MP_IP>/api/v1/fabric/modules)
• Validate that the 'repo-sync' is successful on all three nodes
• Please verify that the '/repository' partition contains both the "from" and "to" version directories.

Additional Information

Refer NSX-T Document Backup and Restore During Upgrade. 
If the symptoms aren't matching, Please refer KB article 326173