The file may not be completely match with the specified file-formatting. This could happens if the file wasn't created by a desktop Excel else any other tool (even the online Excel isn't identically with the desktop Excel). Some tools could handle respectively ignore such deviations and some are rather strict. Therefore try to change the source of this file.
Thanks Marcus - I'll follow up with the guys who provided the file - it should just be the desktop version that created it, as we are one of those companies that lock everything down, but perhaps its a really old file that they keep saving on top. Cheers.