Sounds like it could be related to NeatHtml. To check, temporarily change:
<add key="UseNeatHtmlForXSSPrevention" value="true" />
to:
<add key="UseNeatHtmlForXSSPrevention" value="false" />
in the Web.config. If that makes the problem go away, it's related to NeatHtml.
What version of mono are you using? I think there was a bug in an earlier version mono that caused such an error, so you might try upgrading.
--Dean