I was having this issue on my iPhone and finally was able to resolve it by resetting Excel. I assume this might work from any of the Office apps on iOS.
- Go to Settings
- Scroll down to Excel
- Reset Excel
- Turn on Delete Sign-In Credentials
- Back out from screen (not sure this is required)
- Open Excel and then terminate the app
- Restart Excel and you should get a fresh sign in page
Hope that helps, not sure if this thread was iOS or not...
Karl