Error code 64 is a Microsoft error that means the specified network name is no longer available.
There is a network issue between the NDS Migrator console and the target share.
This usually relates to connectivity issues, meaning network / rights issues are happening.
Investigate each folder/file individually, has anything changed in the environment?
Check the Event Viewer on the NDS Migrator computer and the target computer to be sure there is no intermittent connectivity issues that may be causing this.
Please check with Microsoft for additional troubleshooting of network issues.