At some point (Lehman) ran out of securities that its (commercial) counterparties would accept as collateral...

That point might have been five years ago! IIRCC, Repo 105 only accounted for around $50 billion of the >$300 billion hole that suddenly appeared on Lehman's books. It may be that "cash for trash" "Special Investment Vehicles, (SIVs), courtesy of the Fed, accounted for the rest.

Gotta love the multiple meanings of "accounted." And who says the financialization industry doesn't have a brutal sense of humor. A long term repo written around trash and held by a subsidiary or the friendly Fed is a SIV!

"It is not necessary to have hope in order to persevere."