Office 365 strange program behaviour.

I haven't posted in here for years, but really need help now.

Customer has Office 365 on Windows 8.1. All programs within it start up with the "Microsoft Word/Outlook/Excel etc has stopped working" message. I've seen this dozens of times before. Usually its a duff Add In, or ABBY or similar. But not this time. I've done everything I can think of, but still the problem persists. Oddly, when logged on as a guest or after creating a standard user, access to all Office progs is back. I've even set up Outlook again for him this way, but change the account back to Administrator, and it buggers it up for good again.
Its not a virus, or malware or rootkit. I've checked for all. PC is a CAD machine, and otherwise working perfectly.
I've done the full, clean uninstall and reinstall. Twice. I've also installed his old Office 2007 to try it out, and that does work. Cant load his pst, but it does work.

Any ideas?

Almost a year ago exactly this situation was happening due to update KB2880987.

https://community.office365.com/en-us/f/172/t/251109