I don't know, but just from anecdotal experience (I openly admit it's anecdotes), the B-17 was known to have major chunks blown out of it and remain flying in the worst of damaged conditions. I've heard almost no similar examples of the Lancaster.
EDIT: Consider the B-17 had a much heavier max bombload than it carried. It was between 12000 lbs and 17000lbs if I recall. The choice for a lighter load was a decision made, not a restriction. So while I'd go with "damage" to answer your question, it doesn't rule out "loaded weight" either.