If you manually edit the configuration file (e.g., to add a new vehicle segment) and introduce a missing closing tag, an unescaped ampersand ( & ), or a duplicate attribute, the XML parser throws a silent error. TecDoc reads this as "configuration file malformed."
The configuration file contains a connection string (e.g., Server=localhost;Database=TecDocData;UID=tecdoc_user;PWD=... ). If the server name changed, the database was renamed, or the password expired, the client cannot load data.