Reporting the same issue here with regards to the failing to fetch artifact issue.
Another observation is that when the plugin index is refreshed (for iXsystems plugins) on the https://your.truenas.server.here/ui/plugins page, the Nextcloud plugin shows “Version: N/A” when it used to say “23.0.0”.
I upgraded to Version: TrueNAS-12.0-U7 but I am almost certain I saw the same issue on the previous release build (U6).
Can anyone please confirm this issue exists on that release version?