TYPO3 backend asks for password of SU user when SU backend session has timed out
When you keep a tab open with the TYPO3 backend where you are logged in and put your computer to standby after some time the session times out.
On resuming from standby and revisiting that tab you are asked for your password only and can resume the session. So far so good.
If you have switched to another backend user in the TYPO3 backend and revisiting such a resumed tab you are asked for the password of the user you switched to (which of course you do not know). Instead the password of the original admin user should be requested.