Microsoft has updated their post explaining why some apps such as WhatsApp have been unable to be installed, saying the issue, which is supposed to be affecting older Windows Phones and newly published apps, has now been fixed.
Mazhar Mohammed wrote:
UPDATE (16 AUG): We fixed the digital certificate problem and last evening resumed publishing new apps and updates. It will take a day or two for the repair to fully deploy and newly-published apps to begin appearing in Marketplace again. If you experienced errors, I want to emphasize that you donâ€™t need to do anything to start downloading new apps or updates once the repair has finished rolling out.
As the update mentions, the results of the fix may take a day or two to roll-out, and certainly my attempts to install WhatsApp continued to fail, but hopefully will be sorted in a few days.