In logging in the Microsoft service with COOP account, the troubles you mention occur in using KUMOI.
In cancelling the log in with COOP account, it will be available.

1)
Please sign out the system in case you log in https://portal.office.com with COOP account.
In case you can't sign out, please try the operation 2).

2) If you clear the Cache of your browser to connect to KUMOI again, it will be available.
　 http://www.iimc.kyoto-u.ac.jp/en/services/general/faq/general/post_159

If you install several browsers in your PC, please access with another browser to access from https://mail.st.kyoto-u.ac.jp, it will be available.
（Example：Using chrome at the same time in using IE.）