I don't know why it happens but my form for some reason does not post properly. I am very sure the form contains the required authentication token:
<form action="/account/complete_verify_email/4df78710-e27b-4527-bd2d-71cd2e9a1271" method="post"
<div style="margin:0;padding:0">
<input name="authenticity_token" type="hidden" value="341b07dfa682ed11a045e19956e2e3f5f499e8cb">
</div>
<input id="password" name="password" type="password">
<input name="commit" type="submit" value="Continue">
</form>
Note the authenticity_token is there. What happened?
Update: And YOU KNOW WHAT? Quitting Safari and restart again solved the issue... but I can't think of what kind of intermittent behaviour could cause such problem.