Certain apps not updating on Mac App Store with "temporarily unavailable" and "could not complete your request"

Certain apps on the Mac App Store are not updating (apart from iTunes and system updates), these include Pages, Numbers, Shazam, Evernote, OneDrive, etc.

It gives the following errors each time and refuses to update.

This item is temporarily unavailable. Please try again later.

We could not complete your request. There was an error in the App Store. Please try again later.

I have macOS 10.12.2 on MacBook Pro mid 2015.

I had the same issue and resolved it by signing out (Store > Sign Out) then signing back in again.

As suggested by the other answer, I signed out. From there I got:

This Apple ID is only valid for purchases in the German iTunes Store. You will be switched to that Store.

After this it worked.