Every time I try to install a Plasma theme or Addon, a notification always appears saying “Unknown Open Collaboration Service API error. (0)”.
It’s an upstream issue.
Sure, up and down like a yoyo. It is frankly ridiculous, and makes KDE look like a cluster fork. It needs stripping out - a more straightforward solution ‘Get New Themes’ opening the KDE website in a browser would be FAR preferable.
Just give it up - download straight from the website and use OCS-URL to handle it.
The best thing about using OCS is that, when you install, you get a dialogue which offers to ‘open’. If you select that, it opens the actual installation folder - which you should then Bookmark - then you’ll know where they are.
To ‘Uninstall’ you can simply delete from that folder - and so there’s no more mystery, and no worries about installing 100 themes and deleting 99 of them.