Thanks for such a prompt suggestion.
1. Autocorrect does not touch either "," or "%".
2. The only add-in that is enabled is the "Get Started Tab" mscoree.dll. I removed it and restarted Excel, with same results as before.
3. Language setting is "English (U.S.)" as it has always been.
Note that the reported problem occurred recently; previously Excel behaved just fine and as mentioned earlier created sheets have formulas working properly (but if I try to edit them they fail the Excel formula edit).
Additional testing confirms I can create formulas successfully AS LONG AS a comma is not needed. For example, =average(A1:A10) works just fine, but =average(A1,A10) fails BECAUSE WHEN TRYING TO SAVE EXCEL BUILDS IT AS =average(A1%A10) and that fails the formula edit. The simpler =average(5,4) gets created as =average(5%4) then fails edit too.
I just tried creating a (badly sytaxed formula) =average(A1%a10) (hoping Excel might understand this) but that fails the edit too. So it is building wrong, putting out wrong help text, BUT, editing properly!