If you set it to trust the content then it isn't even using NeatHtml so something else must be causing the error when it tries to process the xml with the xsl.
Nothing has changed in a very long time in the xml/xsl feature so I'm not convinced it is an upgrade problem. It could be something fixed or changed in an update to the .NET framework or it could be a problem with the xml or xsl file.
I would try to get the error detail and google it.
You might also check if caching is enabled in the settings. Even though you changed it to trust content it could be showing a cached version of the output from before that setting changed.