UnEXCELlent jumplists on Windows 10

I was seeing very odd behviour with Excel (pinned to Taskbar) jumplists under Windows 10. Only Excel, of all the Office products, was showing this behaviour. .xlsx files, accessed locally, were placed into the jumplist, but without the Excel icon. Clicking on the jump list entry produced the message "The item that you selected is unavailable. It might have been moved, renamed or removed. Do you want to remove it from the list?". The file, of course, was still in place. This only happened for .xlsx files; other Excel file types on the jumplist, such as .csv and .xls files worked as expected. .xlsx files accessed via the OneDrive folder also behaved correctly.

Even more weirdly, if I moved one of these "missing" files, deleted the jumplist entry (which stayed on the jumplist), then clicked it again, the .xlsx file opened successfully in its new location. Then it would cease to work, as before.

Only my user on the machine was affected. If I logged in as Administrator, Excel jumplists worked as expected.

The web provided a number of remedies. I tried all these to no avail:

• Turned off jumplists globally, from the Settings->Personalisation->Start settings, then turned them back on. As soon as I started Excel, all the old jumplist files returned.
• Deleted the jumplist files, from %appdata%\microsoft\windows\recent\automaticdestinations. You can't navigate to this location using Explorer, you need to type the pathname as a Run command. Same effect as before.
• There was this thread, which fingered DCOM as a potential culprit. Didn't help.
• Removed Dropbox. No change.
• Using regedit to delete all the Excel FileMRU values I could find. No change.
• Turned off Windows Sync. No change.
• Deinstalled Office 2019 using the Microsoft Support and Recovery Assistant, then re-installed Office 2019. No change. I lie; Excel files on OneDrive now failed as well.

There was only one option left I could think of; creating a new user profile and migrating to that. My existing user profile was over 30GB; perhaps this had something to do with the issue. I first had to convert my existing user to a local account, then I could create a new account based on my Microsoft account login name. Excel jumplists for the new user worked.

To effect the migration to my new user, the following steps were required:

• Connect OneDrive and create junctions in the new user's Documents folder to the OneDrive equivalents.
• Re-setup Outlook with email accounts.
• Install WSL (Debian flavour) and added the following packages: gcc, make, pk-config, libbsd-dev, libncurses-dev, python3.7, ssh-client and dc.
• Copy local-only files and folders from old to new user: dotfiles, .emacs.d, WSL .ssh/ files, StudioOne folder.
• Re-configure Firefox with pages opening in new tabs (about:config), removing default tabs (about:config and chrome/userChrome.css).
• Windows 10 configuration elements, such as the Start tiles, File Explorer settings, re-pin applications to the Taskbar, re-add local Tasklets, connect up Calendar (and People) to Google.