When trying to register a JDBC connector with externalised secrets (e.g. connection.password) the validation fails and the endpoint returns a 500. I think that the problem is that the config transformer is not being invoked before the validation so trying to exercise the credentials against the database fails. I have checked that publishing the connector configuration directly to the connect-config topic to skip the validation and restarting the server is enough to get the connector working so that confirms that we are just missing to call config transformer before validating the connector. Please let me know if you need further information.
I'm happy to open a PR to address this issue given that I think that this is easy enough to fix for a new contributor to the project. So please feel free to assign the resolution of the bug to me.