On a sys_user record there is a UI Action titled "Notification Preferences". Expected functionality is that clicking the link will open a dialog window where you can modify the notification preferences for the sys_user record you were viewing. instead, the dialog modifies the logged in user's preferences.
This UI Action is intended to be used only in UI15 as per ServiceNow Development. UI16 is not supported.
Please revert any changes made to the UI Action and switch to UI15 if you wish to use it.