The problem most probably is caused by the Login control destination URL. If a destination URL is not set, the control tries to redirect to the cmsentrypoint and this causes the error.
To fix this, you have to set the DestinationPageUrl
property in the Behavior
section of your login control.
Please note that there is a tiny bug when setting it using the "select" button: the change in the property value won't be saved if you don't manually type the URL (you'll see it if you choose it from the site map, but when you click "Save changes" it won't be saved). You have to type the URL manually and the changes will be saved.
Hope this helps.
the Telerik team