Cant download/install software from AppStore / error -45017

After installing Mojave after I got my MBP back from repair, I cannot install any products from the AppStore any more.

After clicking on the "get" button and waiting for like 2 secs the following error message appears as a pop-up:

An error occurred during purchase.

The operation couldn’t be completed.

(ADIErrorDomain error -45017.)

I've tried re-starting, re-logging in into the store. Nothing helps. Also this occurs with any app, including Xcode (which is the actual thing I wanted to install)

Can anyone help?

MBP retina, early 2015,

macOS 10.14.2