[Plugin: W3 Total Cache] logging out automatically

When-ever I try to clear the cache, I am logged out of my wordpress, and when trying first time to sign again,it comes as wrong username,

then i try again to login in and then the login page appears with no errors
but the url is having some weird things
http://mydomain/wp-login.php?w3tc_note=flush_all
this is repeated until i change the url from
http://mydomain/wp-login.php?w3tc_note=flush_all
to
http://mydomain/wp-admin

After changing the URL, I am able to log in.

Because of this my wordpress dashboard is working too slow.
Whats wrong with the cache plugin