Raul,
I wrote you a reply here a couple of days. Now I don't see it. I hope that's because there was an error or I didn't post correctly, because it would be a real shame if it was rejected by moderators here.
Anyway, the gist of my reply was that your error does not sound like it closely matches our own, and you need to be careful to make sure you hit the 'Submit' button after you select days on your calendar to block.
Also, a good tip from another host here is to also manually set a very high price on days you block. For example, $2,000. Then if the days get unblocked somehow, no one will reserve them at such a high price.
Regarding the cancellation case you mentioned, I think you should start by trying to replicate your unblocking issue. Take screenshots to record your findings. Then contact someone at a higher level within Airbnb if you feel you have not been treated fairly.