OK. Finally the issue has been resolved.
First of all, I have read the thread that was mentioned before but I thought it was strictly related to 802.11a and not to 802.11ac. Fortunately I was wrong. I decided to give it a try and after disabling Quiet IE, Intel 7260 client was able to connect in 802.11ac standard.
What is more, I decided to test it with 802.11k so I enabled Quiet IE again and disabled 802.11k as suggested and it worked too.
So the issue can resolved by disabling Quiet IE or disabling 802.11k.
Thanks guys for your suggestions.