ASSERTION FAILED FAILED TO FIND PARENT ELEMENT (name of the dropdown control) 

I got this error after post back of modalpopupextender ajax control.

I had some cascading dropdowns on the parent page which were hooked on to dropdown controls.

Resolution 

these dropdowns were set visible false on postback of modalpopup in server side code.

Hence when the parent page did a post back it was not able to find these controls.

So, I made the dropdowns visible true in server side and used javascript to hide them instead.