I have a problem connecting to my Oracle database from my VB.NET application. I am using the System.Data.OracleClient namespace. When calling connection.Open() with specifying the correct connection string (see below), I get the error ORA-12541: No TNS listener. I have tried following things:

• Installing the Oracle client on my development machine
• Checked the listener with lsnrctl on my database machine – listener is running and ready
• Tried tnsping from the development machine to the database machine – it succeeded

I have tried the following connections, both failing with the same issue:

• Data Source=(DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.10.127)(PORT = 1521)) (CONNECT_DATA = (SERVER = DEDICATED) (SID = xe)));Persist Security Info=True;User ID=user;Password=pwd;
• Data Source=XE;User ID=user;Password=pwd

where XE is defined in tsnames.ora as follows:

XE =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.10.127)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SID = xe)
    )
  )

As mentioned above, both tnsping 192.168.10.127 and tnsping XE succeed.

I am using a Oracle 21c Database, with .NET-Framework v4.8.

Connection to the database via sqlplus command does work as well.