app store tabs greyed out

After updating an iMac (27-inch, Late 2012) to High Sierra, the App Store's tabs are all greyed out; I cannot select 'purchased' or 'updates' etc, even using the command +2 keyboard options (there are greyed out under the drop down menu too). I have signed out and signed back in, deleted the App Store plist files etc, no luck. Everything else seems to be working under High Sierra except this. Any suggestions on how to make the App Store functional again?