Is this for hostingtropolis.com?
That domain has canonicalization issues, which can cause troubles with cookies.
Cookies for the www and the non-www domains are seen as separate and unrelated cookies. When a visitor clicks a link that takes them from the www to the non-www version of the page (or vice versa), they are switching to a different set of cookies.
I'd suggest picking either the www or the non-www version of the domain and using a 301 redirect to force the choice.