Also, you can just manually enter some definitions for each of the images. The critical thing to get perfect is the image paths, they are case sensitive. If the image is resizable, pick resizable. If it was set to win7 on the old box, make sure it’s set to win7 on the new one, and so on.

Ubuntu 12 is old. I’d recommend a migration to a new (built from scratch) Ubuntu 16.04 server.

I searched the forums a bit for some older threads, I know there are many more than what I found - probably due to the titles of the threads not being appropriate. But here are some, and some related wiki articles for migrating a broke box to a new one:

https://forums.fogproject.org/topic/9200/problem-trying-to-update-from-1-2-0-to-1-3-0/7
https://forums.fogproject.org/topic/8905/procedure-for-migration-from-1-2-0-to-1-3-0-rc/4
https://wiki.fogproject.org/wiki/index.php?title=Migrate_images_manually
https://wiki.fogproject.org/wiki/index.php?title=Troubleshoot_MySQL#Manually_export_.2F_import_Fog_database

#wiki worthy we need some official migration documentation. I can probably have an article put together in a couple of days.