Apple - App Store on Mojave: "We could not complete your purchase. cancelled"

From https://ericasadun.com/2020/05/22/broken-app-store-downloads-on-mojave-we-could-not-complete-your-purchase/:

• Quit App Store
• In Terminal: open "$TMPDIR/../C/com.apple.appstore/"
• In Finder: trash everything in that folder including any pending updates / stuck items.
• In Terminal: killall -9 appstoreagent
• Relaunch App Store
• Done

If you feel confident with Terminal you can also quit the App Store and then run

rm -r "$TMPDIR/../C/com.apple.appstore/"*
killall -9 appstoreagent 

I fixed the problem by opening iTunes and signing in there. As soon as I signed into iTunes with the same account as the App Store, the problem went away.

The trash 'pending updates / stuck items' answer didn't work for me. Nor is it convenient for everyone to just upgrade to Catalina.

What did work for me, was to connect to the Internet via a different network. I temporarily switched from our broadband (via our WiFi router) to 4G (via my iPhone's personal hotspot), and voila, app updates started working again.

Wondering if this could be a DNS related issue, I tried switching back to broadband and tweaking the Mac's DNS settings (System Preferences > Network > Advanced > DNS). After a few failures, I found success using 208.67.222.222 as the DNS server.

That doesn't explain why other perfectly good name servers (like 8.8.8.8 and 208.67.220.220) failed but life is too short for me to worry about it. If you're pulling your hair out over this issue, these things are certainly worth a try.