Sorry I don't know a quick way to clean it up.
XHTML is a newer (and stricter version) of markup that is meant to be a replacement for Html. It is better for accessibility and future preparedness of your site but there are still plenty of sites on the internet using some flavor of html so there is nothing especially wrong with using html but it is better in my opinion to get on board with the newer standard and try to achieve compliance
A couple of links:
http://www.webstartcenter.com/howto/xhtmVhtm.php
http://www.webstartcenter.com/howto/dtd.php
mojoPortal is designed to be Xhtml compliant but there is nothing preventing you from entering non-compliant markup into the content system such as through copy paste. Also nothing in mojoPortal prevents you from deciding to use html instead. The DOCTYPE is specified near the top of the layout.Master file in the skin and you can use any DOCTYPE you choose, see the link above about DOCTYPES or DTDs.
The xhtmlConformance tag in the web.config is for configuring the rendering of ASP.NET server controls. Setting it to strict as it is by default in the mojoPortal web.config tells the server controls to render themselves using xhtml compliant markup.
The only problem with the flash control is that it is added to the page with an embed tag which is not a valid tag in Xhtml. It has been deprecated according to this page (
http://webdesign.about.com/od/htmltags/p/bltags_embed.htm), so things like flash should be added to the page with object tag instead:
http://webdesign.about.com/od/htmltags/p/bltags_object.htm
When you are able to produce this error in the browser and it goes to the error page, does it only happen if using turkish language or does it happen also when using english?
Still I think the problem might go away after you clean up the markup.
Hope it helps,
Joe