Can't install updates from macOS App Store

I'm on a new Mojave install and I can't install updates to Apple suite programs (Garageband, Pages, Numbers, Keynote and iMovie). I've updated macOS and Xcode command line tools successfully from the Software Update app. When I open App Store, it says that I'm not signed in. (Logga in means Login in Swedish).

I try to login, but each time I enter my Apple ID password it just loads and exits the login prompt, without switching to logged in state. Note that I've logged into iCloud and iTunes successfully.

When I try to update the outdated apps. The circle loads for some minutes, and then the Update button returns.

I've tried some different tips that I've found online, such as this, and this.

I have the same exact problem. I'm staying in a long term hotel and I had a couple of problems with their WiFi so I figured it had something to do with this. Yes, it was related but then something messed up around with my Apple ID. Then I could not even log in iTunes and AppStore and even my phone started acting weird.

The solution for logging again was typing this on the terminal:

defaults write com.apple.appstore.commerce Storefront -string \
    "$(defaults read com.apple.appstore.commerce Storefront | sed s/,8/,13/)"

(Source: Appstore error 500 (com.apple.commerce.client))

The problem is that my downloads and updates still never start and according to Apple Support everything is fine and it is just my network blocking downloads from AppStore! But I've tried 4G and a VPN interface and it is still the same!

This is also a problem in 2021 with macOS Big Sur. The design is a little different and it actually pops up an error message, but it is essentially the same issue. Here is what I did to fix it.

Close the App Store app then open the folder that holds the temporary files for the App Store and delete them. Here's how.

Open the terminal app and write:

open $TMPDIR../C/com.apple.appstore/

This will open a finder window. Use it to delete the files in the folder. It might ask for your credentials.

If you know what you are doing (and have the privileges for it) you can delete the files directly from the terminal:

rm -rf $TMPDIR../C/com.apple.appstore/*

Answer y (for yes) to the question.

Now open the App Store app again and click the update button again.