I've managed to narrow this down some more; the problem only arises when buildsurveysession() is called from the following code in index.php. The code segment is

// Let's do this only if
// - a saved answer record hasn't been loaded through the saved feature
// - the survey is not anonymous
// - the survey is active
// - a token information has been provided
// - the survey is setup to allow token-response-persistence
if ($thissurvey['tokenanswerspersistence'] == 'Y' && !isset($_SESSION['srid']) && $thissurvey['anonymized'] == "N" && $thissurvey['active'] == "Y" && isset($token) && $token !='')
{
// load previous answers if any (dataentry with nosubmit)
$srquery="SELECT id FROM {$thissurvey['tablename']}"
. " WHERE {$thissurvey['tablename']}.token='".db_quote($token)."'\n";

$result = $connect->GetOne($srquery);
if ($result !== false && !is_null($result))
{
    $_SESSION['srid'] = $result;
}
buildsurveysession();
loadanswers();

}

This happens after I click "Next" on the welcome page; presumably the issue is that the URL loses any query string at this point and simply addresses /index.php.

Presumably the offending line of index.php

$_SESSION['ls_initialquerystr']=$_SERVER['QUERY_STRING'];

should at least do an isset() on the $_SERVER['QUERY_STRING'], but I don't know enough about what the code is doing to know if anything else needs to be done.

Fixed in rev 10454

New 1.91+ version released