Confirming on *Ubuntu* 18.04.
If I start 'Software & Updates' from within synaptic then I too get an
empty dialog but if I start it as a stand alone application then I am
able to log-in with my Ubuntu SSO email and password. I then see a
further message in German: "Benutze einen anderen Account...".
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Update page - Sign In is in German, unable to proceed
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