It all becomes clear, greg.
![[Linked Image]](https://www.outdoorking-forum.com.au/forum/uploads/usergals/2011/01/full-2772-354-worn_ride_on_belt_dscf1582.jpg)
I've blown up a piece of your picture to clarify what the problem was. That belt wore until it bottomed in the V-belt pulleys, then it slipped because it was no longer jamming in the pulley the way a V-belt must if it is to drive anything. It didn't matter how much tension was on it - once it bottomed, it slipped - then it cooked itself and cracked.
All this is shown by the extreme shine on the bottom of the belt. It cannot get shiny like that if it is working properly, because it doesn't touch anything.
You need to check how deeply your new belt sits in the pulleys. Most likely the pulleys are worn as well as the belt, though not as much as the belt. You will now need to keep an eye on the belt's position in the pulleys. At some point those pulleys will probably require replacement. This is fairly normal: grit gets on the belt, resulting in some minor slippage. The grit-coated belt acts as a belt-sander on the pulleys, and the grit-coated pulleys at as disk-sanders on the belt. They are wearing parts, unless they are fully enclosed to keep grit off them.