More reasons to encode the return URL: http://lussumo.com/community/discussion/7061/returnuri-and-modsecurity/

Here is the fix for the url encoding bug:

Index: src/library/People/People.Class.Session.php
===================================================================
--- src/library/People/People.Class.Session.php (revision 95)
+++ src/library/People/People.Class.Session.php (working copy)
@@ -34,7 +34,7 @@
if ($this->UserID > 0 && !$this->User->PERMISSION_SIGN_IN) $this->End($Context->Authenticator);
$Url = AppendUrlParameters(
$Context->Configuration['SAFE_REDIRECT'],
- 'ReturnUrl='.GetRequestUri());
+ 'ReturnUrl=' . urlencode( GetRequestUri() ) );
Redirect($Url);
}
}

People r97