Точка и запетая ;
в края на командата ми причини същата грешка.
cmd.CommandText = "INSERT INTO U_USERS_TABLE (USERNAME, PASSWORD, FIRSTNAME, LASTNAME) VALUES ("
+ "'" + txtUsername.Text + "',"
+ "'" + txtPassword.Text + "',"
+ "'" + txtFirstname.Text + "',"
+ "'" + txtLastname.Text + "');"; <== Semicolon in "" is the cause.
Removing it will be fine.
Надявам се да помогне.