I would perform the following checks to resolve the 'Error: authentication error: No valid Kerberos authentication for user':
* I would make sure to use a minimum of 2.2.7 version of the Npgsql provider.
* If the User Name present in Active Directory is used to connect to Virtual DataPort, then sometimes while using ODBC client connections, Active Directory may block Virtual DataPort from utilizing the account provided.
* To resolve this, it is needed to adjust the value of userWorkstations attribute in Active Directory in order to enable Virtual DataPort to access this account.
You could take a look at the Knowledge Base (KB) Article [Kerberos configuration and troubleshooting](https://community.denodo.com/kb/view/document/Kerberos%20configuration%20and%20troubleshooting?category=Security) and [Using Kerberos Authentication](https://community.denodo.com/docs/html/browse/7.0/vdp/developer/access_through_an_ado.net_data_provider/using_kerberos_authentication/using_kerberos_authentication#using-kerberos-authentication) section under the Virtual DataPort Developer Guide for more details.
Hope this helps!