Hello Loris,
you are using the jTDS driver. So your problem might be related to this post regarding handling of TINYINT types. Maybe you can try the Microsoft SQL Server driver. To use this driver you have to install it separately in KNIME due to license restrictions. To do so open KNIME and go to File->Install KNIME Extensions… In the window search for SQL. Then select the KNIME Microsoft JDBC Driver for SQL Server extension. Click Next etc and restart KNIME at the end.
Once KNIME has restarted you should be able to select Official Driver for Microsoft SQL Server in the Driver Name option.
If this doesn’t help and it is related to a numeric column you could try to cast the column before reading it into KNIME via the DB Query node.
Bye
Tobias