User had found that there migration to Update 16 was failing on every attempt. the same error message would appear which was misleading:
"Migration failed during backup process.
Check you have enough disk space left on your project disk etc."
The actual problem had nothing to do with disk space. The original project included a custom SRS with GEOID that was not added to the CC installation.
After copying the files to the correct location the Migration succeeded.
User requests that more helpful guide would be useful when migrating.