fivestar only save one vote

Firstly, I'd like to thank you for this great module.

I've found a problem while implementing a fivestar-vote on my nodes.

My situation:
drupal 6.13 => clean default install
fivestar 6.x-2.17
votingapi 6.x-2.1

and, NO any other contribute module.

I enabled fivestart for the default content type "story", and then the fivestar shows up, it's cool.
And then I added a user "test1", and configured the permission that "test1" can vote now.

Then the problems came out, fivestar can only save the "admin" vote.
I use the "admin" user in Firefox, and use the "test1" in IE.( because I want to avoid a cookie conflict ? )
I use "test1" to vote, is shows "saving your vote", and then "your vote have been saved", and then "Your rating: 2 Average: 2 (1 vote)". But after I reload the page, my vote disappeared...The text under the stars is "Your rating: None Average: 2 (1 vote)", but the star is still 2 star, that is right.

I've tried several times, every user can vote correctly, but only the display of "admin" is right.
And, only the last vote value is saved.

Is anybody know something about this? Thanks!