Hello Sterling,
thank you for your reply and advice.
I followed your instructions in the link and unfortunately it didn't solve the problem.
I must mention that the iTunes succeeded to transfer purchase apps, however backup still fail.
at this stage I tried to install iTunes on a different laptop, unfortunately the 2nd laptop showed the same issue.
(both laptops are not the same model).
and - when I did backup in one of these laptops to an iphone 4 (another one) it succeeded.
its a new iphone 6, what could be the issue?
Or