Bad redirection after registration for free event
Its a known bug, and there is a patch to fix it. You can follow the following steps to fix it:

Open file: civicrm/CRM/Core/Payment/PayPalImpl.php 
Search for:

$sub = empty( $params['is_recur'] ) ? 'xclick' : 'subscriptions';

Replace with:

$sub = empty( $params['is_recur'] ) ? 'cgi-bin/webscr' : 'subscriptions';

It should be around line number 404

Hope it helps.