This isn't unlikely at all. Different numbers of chromosomes have been found in DNA extracted from Neanderthals. If Neanderthals -- which we know were a specific breeding community -- had that much variation, why couldn't others in the human family tree?
Actually not all humans have the same number of chrosomes. It's true that the most common variances are associated with retardation, not anomolies prevent reproduction.
Good grief! There are different number of chromosomes in MANY humans TODAY (such as in Downs or Klienfelter's syndromes). The issue is not whether differing numbers of chromosome pairs occur. It is whether it is possible to propagate an extrachromosomal or subchromosomal genotype and thereby create a new species.
Thank you for demonstrating that you in fact do not understand the topic.