2002 t1n keeps throwing serpentine belt

Hello, I was driving my 2002 sprinter 3500 today, it was operating as normal on my trip out. On my way back, while travelling in a straight line, I heard a thwack, the battery indicator lamp came on and after pulling to a stop I found that the serpentine belt got thrown off its tracks. Power steering stopped working. I put the belt back in place by hand using a diagram I found, and restarted the engine. I was able to move about 30 feet before the belt popped off again.
I put it back on by hand a second time, then restarted the engine. The belt was immediately thrown.
I found a small piece of melted plastic stuck to what I believe to be the water pump's wheel (it has a thin rubber layer on the wheel, the wheel also moves around a bit)
The only guess I have so far is that one of the wheels is seized and keeps throwing the belt. The belt is in tact.
Any input is helpful. I have to drive it a half mile as it is to get the sprinter back to my property soon.
-mellowhobo

I would check the two idler pulleys. They can have the outsides sheer off and make the diameter small enough that the tension is not enough to keep the belt on. I would also install a new tensioner. It's cheap and easy and if you've never done it before and it has high miles it's worth doing.

I would check the alternator pulley too, mine seized and snapped the tensioner off it's bolt