It should be noted that there are organisms where populations differ in their karyotype while still being compatible. This is quite common among rodents, where population of the same species can be easily identified with their karyotype, but still produce viable and fertile offspring.

So I don't know enough about this particular case to say where exactly is the problem, but just having a different number of chromosomes isn't such a barrier as people often make. Yet, it is still likely related because chromosomes represent organisational units. During meiosis, there are multiple steps where the material is divided. An unequal distribution might possibly be the cause, but again I don't know enough about this particular case and I would have to speculate.