Query strings

Hi! Thanks for this nice plugin, i find it really flexible!
But i have a trouble with query strings that i not understand.
I have a page with a link including a query string:
<a href="/index.php/logon/?redirect_to=/index.php/previous-page/" class="modal-link">Logon</a>
or
<a href="/index.php/logon/?redirect_to=/previous-page/" class="modal-link">Logon</a>
It will not be rendered correctly in the modal box.
I use it for display the logon window (using wp-members, that allow the “redirect_to” query string), and if the authentication is ok, redirect back to the page that i was reading.
Instead if i do:
<a href="/index.php/logon/?redirect_to=/index.php/" class="modal-link">Logon</a>
the page is rendered correctly.
Any thought?
Thanks!