@Jörg Riedmiller
Apologies for the delayed response. Since you are getting a response when attempting to connect with Oracle the issue does not appear to be related to Azure or the networking security groups or firewall.
I found this information on the Oracle documentation
"TNS-12541 or ORA-12541: TNS:No Listener
These messages indicate that the connection request could not be completed because the listener is not running.
Perform the following actions:
Ensure the supplied destination address matches one of the addresses used by the listener.
Verify the listener is running at the address specified by the request.
Ensure the listener is listening on the host and port specified by the request.
Verify the client is pointing to the listener."
Since this is related to the Oracle specific configuration I recommend reaching out to Oracle Support or posting on a forum such as Stack Exchange.
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