Evgeny,
I've tried everything I can think of and I'm pretty sure I've exhausted every possibility...
I went to my working version of the page and made these changes so that I could determine what the changes do. In all cases, the change from the default to a new setting causes a blank control.
I've set ClientRuntimePath in OnInit, OnLoad, and in the client side .ASPX file as a control parameter.
I've used virtual, absolute, and physical paths. I've tried verbose, "~", relative, and everything else.
Nothing seems to work.