I have no ideas about that so far. I've checked the Polls add-on code and it obviously is not included, when the message is being inserted into the database. There are some hooks on deleting the message, but it should not happen when the message is being posted.
I created a new minibb forum in localhost and installed polls to test that but it was ok.. I think because I've got a large DB something is happening with loading the page with new sending page!
When I click on "Post Message" and close the page instantly and reload it.. I can see posted message!!! but If I send it and stay on the page should wait a few seconds to page loads with new post!