Hello Marin,
I have also run into this same issue that many others have reported. I'm also running the latest version of the ASP.NET AJAX controls. I have pinpointed the issue to this block of commented out css:
The css is not in a stylesheet (don't know if that matters). It's within a style tag in the header. In my project, all I have to do to replicate this issue is load the page and click a tab on my ajaxified RadTabStrip. The RadTabStrip is set to update a RadGrid and itself using a RadAjaxManager in this fashion:
But you and I both mentioned, the issue appears to be in the parsing of commented out CSS. If I delete the commented CSS, the error goes away. I hope you guys can resolve this so I don't have to remember not to leave comments in my CSS.