This is also happening to me.
We are using a RadGrid inside a user control that is dynamically loaded.
The RadGrid uses EditMode=Popup which also uses a user control.
The save button inside the popup usercontrol is an imagebutton.
I tried adding the following as suggested in the link.
When it posts the content is empty. Other controls, like RadTextBox and RadComboBoxes are working just fine.
Is this a bug in the latest build as it never used to happen?