When you try to use Outlook 2019, which is included in Office 2019 with @un.tsukuba.ac.jp mail account, you may be asked for your password for multiple times.
Please refer to the following and take corrective action.
Outlook 2019、Outlook365でパスワード入力を何度も求められる(教職員専用サイト)(in japanese)
Alternatively, use Outlook 2016, which is included in Office 2016. (You may be able to avoid this by repeating the password input 10 times or more.)