Microsoft Support found the root cause of this problem affecting Excel and Word on my Dad's PC. It was caused by a third-party add-in called 'Sapro Systems WinCalendar v4', though in your case it may be a different incompatible add-in. Fixing it was a matter of disabling the add-in. For Word, they disabled it by finding the Office startup folder and located the add-in file there - they moved the file out of that folder (which disables the add-in). Sorry, I can't remember the exact path of that folder, but it was under a c:\users\[Windows username]\ sub-folder. There would be an option in the Word front-end to access the Add-ins settings screen (similar to what I have described below for Excel).
For Excel, you should be able to access the add-ins as follows:
1. Launch MS Excel.
2. Create a new, blank worksheet (or open any worksheet).
3. In the middle of the title bar of the Excel window, there will be a search field containing the hint text: "Search". In that field, type "add-ins" (without the quotation marks) into that field and hit the 'Enter' key. This will display a menu of search results. In the search results, select 'Add-ins', which will be located under an 'Actions' sub-heading - this option will be shown with a 'cog' icon indicating that it is a settings/configuration option. That option will open the Add-ins dialog.
4. All add-ins will be listed in the Add-ins dialog. If any add-ins are active, then note what they are (so you can progressively enable them afterwards), untick the ticked check-boxes to disable them and click 'OK'.
5. Close and re-launch Excel.
If that fixes the problem, then the issue was caused by an incompatible add-in. If you had multiple add-ins enabled initially, then you can use an elimination method by re-activating one Add-in at a time to determine exactly which one caused the problem.
