Rear Seat Belt Problem

I have an 06 ML350. I folded down the rear seats to haul some items and when I went to put up the rear seat on the drivers side the middle passenger seat belt seems to have locked and I cannot raise the seat. Not sure if anyone else has experienced this. Not sure if this is a warranty item or not. I lease this vehicle, but take it to an independent shop for scheduled maintenance.
 
Thanks
 
Mark