Oracle 19c has a Bug when the database was installed using a virtual user. https://docs.oracle.com/cd/F19136_01/rnrdm/database-release-notes.pdf Bug 29426320 ORA-28575 "Unable to open RPC connection to external procedure agent" error occurs when running external procedures or Oracle Database Extensions for .NET procedures with a virtual user account. Workaround#1 Restart the Oracle listener service and the OracleClrAgent service using a non-virtual user account. Workaround#2 Reinstall Oracle Database 19c and select a non-virtual user account. Workaround#3 If you require a virtual user account, then set sqlnet.authentication_services to NONE in SQLNET.ORA. Therefore the above mentioned tipp by @ Serguei Sokolov about setting the sqlnet.authentication_services = (NONE) helps and solved the issue for us.
... View more