Hi I had same issue and was breaking my head, I had a custom login control and was trying to implement secure pages on multi site solution, Once user clicked to secure page, it was being redirected to login page, though from their it was always trying to redirect to eg>
Click on "Contact Us" (a secure page)
The link is: http://mytestsite.com/1/login?ReturnUrl=/1/contact-us
Site breaks as it goes to http://mytestsite.com/1/%2f1%2fcontact-us
This solution worked for me:
The problem is that Sitefinity uses outdated scripts in some cases.
If you need to call the Response.Redirect in your update panel you will
have to make a small change in your configuration. Navigate to
Administration -> Settings -> Advanced. In the left panel navigate
to the following nodes Pages -> Script Manager -> Script
References -> MicrosoftAjaxWebForms. Look at the main area. Change
field from "Telerik.Sitefinity.Resources.Scripts.MicrosoftAjaxWebForms.js" to "MicrosoftAjaxWebForms.js" and theAssembly
from "Telerik.Sitefinity.Resources" to "System.Web.Extensions" and
press the "Save changes" button. You can also look at the attached
After this change, you will be able to use your redirects inside the UpdatePanel.