Hi Simon,
I had the same issue with a customer last week. (Win10 + CC2021 / CC2022 + Latest CC App).
You can guess: Restarting PS, the CC App, the PC and even reinstalling PS didn’t help.
I only had the chance to check the system for a few minutes.
There the last two folders (/Plugins/External) were not present at the standard UXP plugin installation location:
%appdata%/Adobe/UXP/Plugins/External
(In case that the first installed Plugin creates that folders, this behavior might be OK.)
However: Adding these folders manually didn’t fix the issue.
I have then pasted the plugin itself manually to:
%appdata%/Adobe/UXP/Plugins/External
And expanded the config JSON here:
%appdata%/Adobe/UXP\PluginsInfo\v1
(I’ve borrowed both resources from a system, where the installation had worked.)
This did the job. But obviously it’s not nice, because the JSON was edited by user and the plugin is not registered properly (e.g. to install/update it via the CC App).
I had also installed the UXP Developer tool on the PC. Using that it was possible to load the plugin without issue.