On pressing the Connect button in Test To Live the following error is displayed:
Progress status: Loading databases, please wait...
Error message: Object reference not set to an instance of an object.
This error can occur when the Live database contains tables that do not exist in the Test database. This has been logged as Problem 5853 and is fixed in version 7.7.1 (released April 2014).
The fix is to manually delete any tables from the Live database that do not also exist in the Test database. Test To Live is going to want to delete those tables anyway as part of the synchronisation so there is no (extra) data lost by deleting manually beforehand. If you are uncertain about what exactly should be deleted please contact your local support provider for assistance.