rsviewerpage.aspx Type Sys.UI._Timer has already been registered

You have configured SSRS on SharePoint 2010. You successfully open report builder and run report, but when you try to run it through rsviewerpage.aspx it stucks.

My suggestion, as usual, look at the logs:
 - ULS;
 - SSRS log (SSRS: How to troubleshoot?)

But in this specific case, I found really helpful look at the F12 Developer tools panel in IE.
In console I found an error:
Type Sys.UI._Timer has already been registered
SCRIPT5022: Sys.ScriptLoadFailedException: The script '{your portal url}/ScriptResource.axd?d={some data}


It turned out that the error in rsviewerpage.aspx is due to configuration in this specific web application:
Check in web.config the following:
 <compilation batch="false" debug="false">

In case debug="true" rsviewerpage.aspx gives you that error. As you may guess, once you set debug="false" reports will run successfully.

I officially announce such behavior as a bug in SharePoint 2010 and SSRS integrated mode:

   Setting in the web.config debug="true" will case an error Type Sys.UI._Timer has already been registered in the rsviewerpage.aspx  and report stucks