That's the correct belt size and Continental is a very trusted brand, only one I ever use. That many years says you more than know how to tension belts by now.

That's one seriously damaged belt. The cover backing is frayed and separating, a sign it might have been wearing against something on both sides, such as riding low in a pulley. If it broke and got caught up in another belt or behind a pulley then I wouldn't expect damage like that to be the entire belt. It's almost like it was running over a seized or wobbly pulley.

I'm guessing it was hanging by a thread for some time before finally letting go. To go from near pristine to that in a matter of days is indeed surprising.

Have you considered the possibility of rodent damage getting part way through one side then the rest hanging by a thread for a few days? Rodents generally prefer wire insulation, but drive belts have been known to be on the menu. They need to wear their teeth down and will go for anything of the right texture.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now