decrypt zoom chat message fails

[Zoooooom]

Dear Community,

unfortunately Zoom is not able to decrypt messages form an external contact person. The message shown is (the system has german language) "[Dies ist eine verschlüsselte Nachricht]", (translated) "[This is an encrypted message]".

A red exclemation mark ist shown right next to it. Klicking on it shows the following Message (the system has german language) "Die Nachricht wird entschlüsselt, wenn der Entschlüsselungsschlüssel erhältlich ist", (translated) "The message is decrypted when the decryption key is available".

The external contact is not within the same domain but the contact request was accepted form both sides and and the status can be seen.

Does anyone know how to solve this issue and let zoom decrypt the messages in the future? Any hints are highly appreciated. Thank you in advance.