Cannot send private message

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That's really frustrating. I tried a couple to times to send a message to some who is in my friends list but I cannot send the message. Even clicking on preview before sending, does not help. The error message that I receive is that the person is not in my friends list even though he is. I click on the person's name before so that the correct name appears in the sending message box but nothing seems to help. Anyone ever encountered this problem before? I even tried to make space in my inbox, but it won't help.