Connection to App Store not possible

I am online (e.g. I'm searching, reading and writing here in this forum - Safari, Google, Enmail, Skype - everything works)
nevertheless since me updating from 10.6.x tp 10.8.3 (around 2-3 weeks ago) I cannot connect to the Mac App Store or update my Software.
AppStore window shows this
(means: Connection to App Store impossible - you need an Internet connection )
note:
I tried this numerous times over the last 2 weeks - I am online App Store thinks I'm not.
I did reset / PRam reset etc.
It even shows an update available, but still remains in the "no Internet" state.
Any idea where I can delete / reset / kill th esettings for the App Store ?
Apple ID is correctly entered in system setting Users tab
thx for any help - Zo

Bump - meanwhile updated to 10.8.4 (downloaded the 10.8.4 Combo Updater and installed it)
still no change...
neither APP STORE nor the SOFTWARE UPDATES can connect , and claim there is no Internet connection. There definitely is an internet connection, as I'm using it to read and post here and can surf / get email as usual !!
one "minor thing", on connection to this site here
https://discussions.apple.com/message/22144659#22144659
Safari claims the certificate is signed by a untrusted source. (see screenshot below) Is it possible that something is wrong with my certificates here on this machine ? Any advice ?
Should I delete certificates (are any locally stored on a OSX machine) and maybe force a reload of a propper newer certificate ? Could this disturb the (hopefully safe) connection to the App Store ?
thx for your thoughts  - zo

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