You can't see the link,you need to register?

[civilengineer]

When an URL is added in my forum "You can't see the link,you need to register?" is written on it,so URL can't be seen. It's written in Turkish,I don't know the exact translation of this term.

"Link görüntüleyemezsiniz,üye olmanız gerekiyor?"

It doesn't happen everytime,but occurs so often.Post must be moderated to get rid of this problem.But same problem can be repeated on same moderated post.

We're using 3.6.0,we have an installed style,there is no problem in default style.We have installed vbseo but it's not in use.
We had also have hide option,but we uninstalled it.