https://www.reddit.com/r/Office365/comments/14570xg/outlook_suddenly_only_opening_in_browser
I recently solved it, at least for my computer! I found another post which suggested “try[ing] resetting the app by deleting the %localappdata%\Microsoft\Olk directory. You might have to delete %localappdata%\Microsoft\OneAuth as well.” Personally, deleting the “olk” file alone was enough–worked like a charm! I ended up deleting the file, but immediately afterword outlook was back to normal. Hopefully this helps you as well.
