I can surf the websites but can't enter my accounts with username and password

I have recently got one samsung omnia i900. I am very happy with the phone but I have a problem when I surf the internet on my mobile phone. I can visit websites but for example when I want to sign in my account on this forum, I can't, because sign-in form is not activated. The websites don't allow me to enter my username and password. Is this related to some settings on the mobile? Has any of you encountered this problem? What is the reason what is the solution? I will be glad if somebody gives feedback on this problem.

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