While exporting budget from nav 5.1 version system hit following error message
The call to member Add failed. Microsoft Office Excel returned the following message: That name is not valid.
Error hitting on report 82 Export Budget to Excel Line ExcelBuf.CreateRange(ColumnDimCode[i]) Line
This is the standard Nav 5.1 report bug and Microsoft has already fixed this issue on 2009 version , the problem occurs when exporting a dimension code that contains blank space (or special characters) , check the screenshot
Update the two lines to Nav 5.1 version object and export again.