I opened a ticket with Microsoft. The tech had me uninstall Excel 2013, but that didn't help. Excel still worked after the uninstall so we used a cleanup tool to finish the uninstall - but that didn't fix it either. Finally, we tried this;
Remnants from the failed version of Office 2013 was causing the issue.
We deleted the 9.5 registry folder founder at the following location
[HKEY_CLASSES_ROOT\TypeLib\{00062FFF-0000-0000-C000-000000000046}\9.5]
... and that fixed the issue.