I've got an Excel XLT BOM template I've been using since the initial version of CS. I now am using CS v1.5.1.
It suddenly stopped making use of my BOM template, and just generates a "plain vanilla" .XLS file without the template configuration.
Made no changes to Excel lately.
Can't figure it out.
Since you have a client waiting I would advise you use CSV export then do some copy-pasting to format as you require in Excel.
Simon also had some issues with Excel BOM export:
Maybe some Office 365 updates are to blame? Dunno.