The code seems to take an empty result into account with if ($r6), but did trigger on an autoresponder in my setup. This resulted in an empty $title but still the UTM parameters where appended. So I think this enhancement is needed:

//Get the campaign title from database
if($ares_emails_id=='') //if link does not belong to an autoresponder campaign
{
    $q6 = 'SELECT title FROM campaigns WHERE id = '.$campaign_id;
}
else
{
    $q6 = 'SELECT title FROM ares_emails WHERE id = '.$campaign_id;
}
$r6 = mysqli_query($mysqli, $q6);

In case of an autoresponder, query the table ares_emails for $title instead of the table campaign.

I have little experience with PHP but as l.php already does a check on autoresponder with: if($ares_emails_id=='') I though I'd extend the code. However, it would be better to have it integrated in Sendy :)

The whole addition would be:

//Get the campaign title from database
if($ares_emails_id=='') //if link does not belong to an autoresponder campaign
{
    $q6 = 'SELECT title FROM campaigns WHERE id = '.$campaign_id;
}
else
{
    $q6 = 'SELECT title FROM ares_emails WHERE id = '.$campaign_id;
}
$r6 = mysqli_query($mysqli, $q6);
if ($r6)
{
    $row = mysqli_fetch_array($r6);
    $title = $row['title'];

    if (preg_match('/\\?[^"]/', $link)) {
        //If the URL has other queries already, then append the Google Analytics query starting with &
        $link .= '&utm_source=Sendy&utm_campaign='.urlencode($title).'&utm_medium=email';
    } else {
       //If not, so these are its first URL query, then append them starting with ?
        $link .= '?utm_source=Sendy&utm_campaign='.urlencode($title).'&utm_medium=email';
    }
}