The time limit to raise a claim of forged/improper/missing endorsement is three years. Your bank most likely did not notify you sooner because they just received the claim themselves.
IMO, the easiest way to resolve this case is to take the check to the lender to obtain the missing endorsement. Then, once you have their endorsement, redeposit the check at your bank.
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It's hard for an egg to become a bird. It's harder for an egg to learn to fly. C.S. Lewis