I suspect that your region/language list-separator choice got changed. It should be comma or semicolon, but it might be set to forward-slash.
This is a system configuration change, not Excel. But close all Excel windows first.
On my Win7 system, I click Start (lower-left desktop toolbar) > Control Panel > Region and Langugage > Additional Settings, and either click Reset or at least change List Separator. Then click Apply or OK.
Win10 navigation might be (very?) different.
Restart Excel, and verify that =6/2 now works.