Updater repeatedly downloads, tries to open, fails and repeats

My son's MacBook Pro, 15" early 2011 running MacOS High Sierra 10.13.6, has an issue with "Updater". As soon as the MacBook boots up it downloads and mounts on the desktop a virtual drive named "Updater". The Updater launches, displaying the Updater icon in the dock that looks like a tilted sheet of paper with an "A" made up of a ruler, pencil and paintbrush in the lower right corner (similar to the App Store icon?). It quickly disappears and then another instance of the Updater installer is downloaded and the cycle repeats. Sometimes a second Updater icon appears in the dock before the prior one disappears, then they might both disappear simultaneously. The cycle only stops after 24 instances of Updater have been downloaded, each time the MacBook is restarted.

If I try double clicking one of the Updater installers it very briefly appears to launch, briefly displays two icons in quick succession in the dock before immediately disappearing.

Watching Activity Monitor, Network tab during this process of 24 cycles, when the Updater installer shows up briefly it displays varying amounts from 4.7 MB up to 7.1 MB received, but the size of (all) the Updater items (in the Finder) is only 139 KB.

Things I've tried:

1) Updated AppleID login, updated all purchased apps

2) Searching the Internet for similar symptoms I found a couple of threads for similar issues, but several years old (and presumably older versions of MacOS, so I didn't find the following items in exactly the same places as in the threads...

Moved the following items to trash: com.apple.iconservices.store, com.apple.SoftwareUpdate.plist, com.apple.storeagent.plist.

No improvement. Any suggestions?