Hello Robert Smit,
I have checked your live url and noticed the error that you have mentioned. When I looked at the page source I saw that there is no RadProgressManager created using the $create statement. This means that there is no RadProgressManager on the ASPX page that holds your TabStrip. Please, move the declaration of the RadProgressManager to that page. I have attached sample project that demonstrates the approach.
We have also developed more robust version of the aforementioned workaround. Here it is (it is in the sample project as well):
prm = Sys.WebForms.PageRequestManager.getInstance();
inputs = $telerik.$(
In addition, we have fixed the issue for the Q1 release (which is scheduled for the next week) .
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items.