Hi Andreas,
it seems as if the Dremio driver does not support the isValid() method. You can force KNIME to use a query to test the connection validity instead of using the not supported API method. To do so open the dialog of the connector node and go to the Advanced tab. In the tab add any valid Dremio query e.g. SELECT 0 that executes successfully as value of the Validation query setting.
You might also want to check the other advanced settings to see if they are compatible with Dremios capacities e.g. if it supports CASE statements and the identifier delimiter. It might also help you to identify problems if you enable the JDBC Logger in the beginning which log all statements to the KNIME log.
Bye
Tobias