Mac App Store not working on macOS Mojave 10.14 Beta 1 (18A293u)
With latest beta 2 update 10.14 Beta (18A314h) issue is resolved.


Update macOS to latest 10.14 Beta (18A314h)
Open App Store
Click on Store -> Sign Out in menu
Quit App Store
Open App Store again
Sign in


After this I could update apps and initiate new downloads.