it does make sense that if the rear pulley is jammed and can't open all the way, lots of belt friction will be developed and, also, top speed would be restricted.
take the rear unit off the bike..(got a puller?) Remove the big nut in back of the unit.
(Unless you have the special puller) Get a big hammer and a big round piece of steel bar. Pound on that threaded part sticking out the end. Don't damage the threads.
If you're lucky, after a sharp whack or two that center piece will pop in (it's a tapered fit).
Immediately, the strong internal spring will pop the thing open..
Now you can examine the internals for damage or something... it's pretty simple.. only 5 pieces total..
The manual shows the exploded view for reassembly. Reassembly might require a couple clamps to compress the spring while putting the nut back on. I can do it with my bare hands..
Before assembly lube it with grease as the manual instructs.