Wow, you really have bad luck. No one can tell you for sure that the 48v isn't causing you problems but personally I dought it. I've been running 48v thought a Currie 24v motor for maybe 400 miles and had no problems. And I run lifepo4 packs and a 50a controller.

Lets separate the problems. The first problem was the nut came loose and you lost the key. While being a PITA it's easy to explain and understand what happened. The new problem sounds likeit's inside the gearbox. I would remove the outer gear and key. Then remove the gearbox cover screws and carefully remove the cover. There is a very light duty gasket between the cover and the box that can be very easy to damage. So go slow with the removal and watch for the gasket to stick to bth peaches at the same time in different places. Once the box is open you can examine the gears and the second key located on the main gear shaft. You can connect power and test run the motor. Be careful the motor can easily jump out of your hand. Maybe bolt it down if you can. If everything is good at this point the only other thing is the motor shaft (the small gear is actually machined on the end of the motor shaft. The shaft could be damaged but I've yet to hear of this happening. Bob