This is indeed caused by ?i=1 added to the query string by some additional code you have on your site. If there are unknown query string parameters as such, the page is not considered the portal front page. You can add this query string parameter as an exception to prevent this by making the following change:
Sources/Portal.subs.php
'wwwRedirect' => true,
'wwwRedirect' => true,
'i' => true,