The template file is not complete.
Explanation
The template file (default: DifferenceBank.xml in the .metadata folder) is not complete. This file was built during the migration process and this file is necessary for the validation.
User response
There are two ways to solve this error. One is to manually restore the file. Obviously the file should have been previously manually saved. The other is to rebuild this file. The rebuild operation is available from the "Migration validation" preference page with the button "Rebuild the template file".