I feel like this might be related to #16 but it mentions a fix in 6.1 and I'm running 6.2. Still, I am pretty sure it's an upstream bug, because I see this same behaviour in all widgets I've added from New Stuff.
The error I see is:

Wrong KNewStuff URI: kns://plasmoids.knsrc//2128047
I am able to upgrade from the release here using the command in #16, so, thanks for the workaround :) Thought I should report this as it's been a while now since 6.0 (and this bug) dropped.