App Store: 500 Internal Server Error

Hi - I recently upgraded to Mac OS High Sierra. Since then, I've tried to update some of my software apps in the App store. Sometimes this has worked - when I clicked on "update all", it readily downloaded iMovie, and a couple of others apps. But when I try again to update all - the update buttons go grey, and then nothing happens. When I try to click on each application individually, I get the following error message:

We could not complete your update.

The server gave an error during download: 500 Internal Server Error.

I've done a lot of the fixes: deleted the cricache.db and ocspcache.db files, fixed the blue highlighted files in the Keychain Access, updated my DNS to Apple's server settings. None of which have worked. I still return to the App Store and it won't permit me to download updates.

I have 2 very long-standing Apple IDs (when, oh when, will they let us consolidate those things!) and have tried logging in and out of the App Store with both of those IDs multiple times.

Any thoughts or fixes? Many thanks in advance.