I see in FireBug the error: 


My question how it is possible to understand where is the problem?

Hello,

 This is normally caused when there's a parser error in the client-side scripts that the RadScriptManager is trying to combine together. You can try the latest version of the controls which contains the most up to date fixes and improvements to ensure all RadCotnrols related scripts can be seamlessly combined together. Additionally you can check whether you use any custom client-side code and whether it is correctly formatted and error free. Finally you can also check out how this works with the Standard ASP.NET script manager on the page to see if it provides more information on the error. 

Regards,
Marin
Telerik

Updated to the last version. Didn't help.
What can be my next steps to find the problem?

Hi,

 The exception indicates that there's a syntax error when the script manager is combining the different scripts into one common. You can carefully check whether you have any custom client-side scripts or settings in the controls that require loading of additional scripts that are not properly formatted and can cause parser error when the script manager is combining them. Additionally you can also try using a different script manager e.g. ASP.NET default ScriptManager control or the Telerik RadScriptManager control. But most importantly double check any client-side scripts that are loading on the page for any syntax or parser errors to narrow down the cause of the problem. You can also try a more simplified version of the page containing as few script controls as possible to easily identify the cause of the problem. Then you can gradually start to add more of the original script controls to see which one exactly is causing the issue.

Regards,
Marin
Telerik

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.

Hello,

This happens because during an ajax request the content of the head tag is also updated and returned in the request response which the browser is trying to parse but fails.
To avoid the problem you can set the EnablePageHeadUpdate property of the RadAjaxManager to false. 

Kind Regards,
Marin
Telerik