I also faced same issued today after downloading latest version. I deleted and installed again then also same issue.
I raised the issue to whatsapp support with all the details and screen shots also but they gave vague response. see below their response which has no meaning.
Question asked by me : - Not able to download videos which I received in my whatsapp window either individually or from other whatsapp group. I tried both wifi and cellular options to download. I have iPhone 6 and this issue occurred after installing latest whatsapp version 2 or 3 days ago. I deleted whatsapp and installed it again if that resolves issue..but no luck. See picture below to see what happens when I try to download videos. Photo downloading is fine.
I rebooted phone also couple of times.
Request #3538720916716
Their response :-
From: WhatsApp Support<support@support.whatsapp.com>
Date: Mon, Aug 21, 2017 at 5:45 PM
Subject: Feedback/Question re: WhatsApp
##- WhatsApp Support -##
Hi,
Thank you for informing us about the issue; it will be fixed in a future version of WhatsApp. Unfortunately, we can't comment on any future timelines, sorry. Thank you for your continued patience and support of WhatsApp.
Regards,
Tom
WhatsApp Support Team
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