The problem reported, was the same as the problem always is; the budget template was not in the correct format.
The first image is the header of the one they were trying to import and the second is the Master budget I exported from GP to show them:
As you can see the former budget template has an extra row in the header; this is sufficient to stop the import working.
Everytime we have had this error reported we have found that something was changed in the budget template: a new header row, a header row being deleted, a column being added (the latter is the most common.
The client removed the extra row and was then able to import the budget.