On submitting the form with disabled javascript, we have an error 500 due to an mistake in the form action.

Line 241 :

<form id="form-{$uniqueId}" action="{$site-uri-prefix}/{$lang}/_plugins/ugc/page/contents/add-content" method="post" >

The URL is wrong, it should be :
{$site-uri-prefix}/{$lang}/_plugins/ugc/contents/add-content