Moderator Control Panel -> Reported posts -> Open reports -> click any report, then 'Close report' button.

Prosilver template has:
<form method="post" id="mcp_report" action="

">

but no U_CLOSE_ACTION is defined in mcp.php, should be:
<form method="post" id="mcp_report" action="

">

file: styles/prosilver/template/mcp_post.html

But: still error - the report isn't closed, so seems S_CLOSE_ACTION url is also broken.