app store unavailable following security update 2015-004 (Mavericks)

Just installed the Security Update 2015-004 (for Mavericks - being on 10.9.5).

When restarted, Mac came up with the App Store (as usually, apparently looking for further updates).

Problem: Window stays bank, just showing the greyish AppStore icon. Even when changing to other categories (Featured, Top Charts, ...).

Tries to fool around, pressed Command-? (to get to AppStore help). Result:

" Help Center can't verify the identity of the server "help.apple.com" " (and showing all the vertificate readings).

Apple forgot that some certificates are 'bad' ?

Here, already the top level certificate is not trusted (VeriSign Class 3 Public Primary Certification Authority - G5). Is this normal - just ignore (aka, check-mark the Always trust) ???