I think I know what's wrong but I won't be able to fix it until later tonight or tomorrow. In the meantime, please code your app as if it were working. I'll get it fixed as soon as I can. Unfortunately I have to go run the DaniWeb meetup group right now! :)
I just went ahead and tested it, and the voting mechanism works by design for me. I'm not able to reproduce your errors. It should be noted that currently only voting is supported and not unvoting. (This will most likely be added in the future). If you attempt to use the API to vote on a post that you have already voted on, or your own post, it will return success: false.
I tried a few times in a row to vote through the API, unvote through the standard UI, and then re-vote through the API, and it all worked as intended.