a: expired certificate
Dialog text:
The signature is invalid: The signing certificate has expired.

a: expired certificate

"Geheimen Team-Schlüssel zum internen Teilen abspeichern." is grammatically correct, but it sound very formal and clunky for a UI tooltip. It lacks clarity, therefore I suggest:

"Upload successful." or "Upload complete." (even shorter) for the title bar are fine :)

engl. Menu Entry: Save Secret Team Key
Tooltip: Save this secret key to share with other team members.

Vorschlag Text:

You have imported a certificate with secret key.

Here are some ideas:

"Exclusive user" sounds a bit odd and could still be misinterpreted. A native speaker would probably say "Are you the sole user of this secret key?“ or (even better and shorter) "Are you the only user of this secret key?"