A chunk of styro foam would certainly cause no structural damage like dislodging the locking pin or what ever, so what did happen?
It could be as simple as the possibility that the door was not latched.
NASA has by it's own words, eliminated the ice theory. (they would know, because the crew viewed the tank on separation and likely saw the missing piece)
I am still looking for the cause. I cannot satisfy myself with the foam theory and neither can NASA.