A typical mistake when automating scripts using session variables is using the static value for the session variable (obtained by recording the sequence of HTTP requests with the Record function) while these session variables are unique for each session; thus an attempt of running such a test script will lead to errors (since other sessions will generate different values of session variables). To provide proper functioning of test scripts in cases like this you need to extract respective session variables from the responses to previous requests (see the description of $UrlParam, $Hidden, $RegExp).