I had a similar problem with my Honda HRN216VKA mower. While mowing (and mulching) the transmission stopped working. Turned in to a push mower. Less than 1 year and 10 hours use old.
I found your post yesterday and dreaded the possibility of a broken transmission. But the problem was actually very simple--once I found the small piece of wood that jammed the belt in the transmission pulley.
The belt and pulleys on the motor shaft and transmission have guides to keep the belt on the pulleys--See part #4 on the diagram. I don't think it would be possible to replace the belt on the transmission without removing the entire transmission and then removing the nuts the attach the guide to the the transmission body. Most parts sellers have pictures (see attached) of the transmission with the belt installed--replacing the belt (~$9) is much harder than just replacing the entire transmission with the new belt (~$50).
A small piece of wood--about the size of a large pea--jammed into the transmission pulley under the belt. The resulting bulge in the belt could not pass under the belt guide. Jammed tight. The pulley would not turn nor would the transmission shaft to the wheels. The wheels turned easily.
After taking the blades off and unmounting the transmission shaft from the mower deck I was able to jiggle the belt back and forth and out popped the little piece of wood. Reassembled everything and back in business.
I've found that it's usually something very simple that causes problems with appliances and small machines. A cherry pit in the dishwasher pump impeller nearly had me buy a new dishwasher until I took the pump assembly apart and out dropped the pit.