It looks like this is the reason:
“All Mac Store Apps Stopped Working Due To Expired Security Certificate” 
<a href="http:&#x2F;&#x2F;techcrunch.com&#x2F;2015&#x2F;11&#x2F;12&#x2F;all-mac-store-apps-stopped-working-due-to-expired-security-certificate&#x2F;" rel="nofollow">http:&#x2F;&#x2F;techcrunch.com&#x2F;2015&#x2F;11&#x2F;12&#x2F;all-mac-store-apps-stopped-...</a>
<a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10554700" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10554700</a>