Cannot connect to App Store since Mojave

Just updated to Mojave and dark mode on our iMac 2017 27".

I can see all my iCloud settings in System Preferences > iCloud > Account Details.

In the App Store, however, I cannot sign in, meaning it asks me for my credentials, but after I hit enter nothing changes and I am still not signed in.

In Books I am still signed in (acc. to the menu "Store"), but I cannot see my purchases, because it cannot access the book store.

I deleted the files in /var/db/crls, reset all the Apple Root Certificates and even ran this terminal command

defaults delete com.apple.appstore.commerce

I rebooted several times, ... but I still cannot sign in at the App Sore,

iTunes and Photos work and I can see my Library and Photo Stream, respectively.

Any help or hint? Thank you!