As I have commented, you should check each url within the htmlData string. You need to make sure each url is accessible without any proxy errors by your .NET assembly. Also search for any parsing tags used by template engines. Here are some that come to my mind:
<? ?> - php
<?php ?> - php
<?= ?> - php
<% %> - asp, asp.net, php
<%= %> - asp, asp.net, php
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