Finally I have decided to upgrade my current High Sierra to Mojave to get newer version of Xcode and also to support the multiple monitors with Targus USB-C dock station. I have heard people had various issues after upgrade to Mojave, so I am a bit unsure it is worth doing it.
It took me about one hour to complete the upgrade. And the first thing that I noticed after the upgrade is App Store is broken!
I tried a few fixes that I googled, and here is the one that fixed my problem: Go to /var/db/crls/ directory, and move the crlcache2.db out of there, then restart the mac. After the restart, App Store works again for me 🙂