I was leery of removing files from ~/Library, but I didn't want to have to completely reinstall Office either, so I eventually did this, and it worked for me. To make it a little safer, save the files rather than delete them. Here is a slight tweak of the original instructions:
(If you use Outlook, I suggest first quitting the Outlook app. See more Outlook comments below.)
From the command line (i.e., in the Terminal app), I did:
mv ~/Library/Containers/com.microsoft.PowerPoint .
mv ~/Library/Group\ Containers/UBF8T346G9.Office .
(Moving both of these folders as I do here ensures that Powerpoint finds it's license when it starts again. If you move only the second folder, it asks for license info.)
Then run powerpoint, and voila, the "insert equation" feature works again, and replaces things like "\alpha" and "\int" with the appropriate symbols. If you want to restore your custom dictionary, then exit powerpoint and in a command line do:
cp -p UBF8T346G9.Office/Custom\ Dictionary ~/Library/Group\ Containers/UBF8T346G9.Office
and overwrite the default dictionary that was there.
Alternatively, all of these commands can be done from the Finder as well. In the Finder, Go, "Go to Folder...". In the box enter "~/Library" and navigate to the folders you want to move somewhere else.
This fix is also reported at:
where they also say: "Please note that removing the UBF8T346G9 folder will also remove the Outlook profile folder." I don't use Outlook, but if you do, then after doing the above and have equations in Powerpoint working again, you may want to do:
mv ~/Library/Group\ Containers/UBF8T346G9.Office/Outlook* .
cp -pr UBF8T346G9.Office/Outlook* ~/Library/Group\ Containers/UBF8T346G9.Office
(edited to incorporate suggestions.)