This is a known issue - by design all RadWindowManager functions like radopen, radalert, radconfirm etc. will always use the RadWindowManager that is first rendered on the page. Since you update your control with Ajax, this RadWindowManager is destroyed during the Ajax call and then recreated again, however in such case, radopen, etc. functions will throw an error as they still search for the original one.
What I can suggest in this case is either to upgrade to a recent version of the control, were we improved the behavior of the RadWindowManager in such scenario, or if you don't want to upgrade - to exclude the RadWindowManager from the Ajax update - for example you could put it on the parent page. Note that in such scenario there is no need to ajaxify the RadWindowManager because you are using its client-side API only
All the best,
the Telerik team