Question Strict standards: Only variables should be assigned by reference

Go here:
www.gameworld.gr/forum

Check the bug that appears:
Strict standards: Only variables should be assigned by reference:
/home/gameworl/public_html//plugins/system/kunenasocialshare/kunenasocialshare.php on line 29

The only thing I did was that I changed the validate-uncompressed.js and removed the code

Object.append(Browser.Features, {
inputemail: (function() {
var i = document.createElement("input");
i.setAttribute("type", "email");
return i.type !== "text";
})()
});

Then after the error appeared, I put the old file back to where it was. But then I saw the above error again.

Strict standards: Only variables should be assigned by reference:
/home/gameworl/public_html//plugins/system/kunenasocialshare/kunenasocialshare.php on line 29

In the meantime a developer was making changed to my website. The bug he needed to fix was that:
"Banners not appearing correctly if a user is logged out from www.Gameworld.gr "

So the question is: Was it my fault that I changed the code? Now I put the correct file there.

Please confirm on how to fix this issue! Maybe if I replace some files it might be fixed. There is an issue with the dates in www.gameworld.gr/forum .

Thank you.

I have the same error after update to version 5.1.3. It is only visible after you are logged in as a user.
What can I do?