Hi Pothen,
Make sure the jdbc jar file is in the ....bin\jdbc folder for the SQL Agent (and the DB Services Agent) - Restart the agent after file are added.
-If you don't want to put the JDBC file in the DB Services Agent, then instead of using the "Test" button - Use that CONN object in a SQL job instead as your test.
-Make sure that the Agent is using/Starting up with a JRE version that compatible with that version of the sqljdbc
Microsoft JDBC Driver for SQL Server Support Matrix | Microsoft Docs
System Requirements for the JDBC Driver | Microsoft Docs
-Also the agent will use the highest version of the jdbc file, so make sure the highest one is the one that compatible with your version of SQL Server.
If after checking/doing all those and you still have the error. I would recommend opening up a ticket with support for them to go over with you in full detail of your setup.