Do the end users have this end user group as their current group? If not, it might be worth a try setting that and trying again. With permissions, check the event log as sometimes they pop up in there and you can spot what needs to be added. I'd make sure the *role* end users are all in have read access to everything in the knowledge area. You could also spin up a query that displays all knowledge articles and see if an end user can view and open those. If so, it isn't a simple privilege issue on the articles.
The Role equivalent for End User is PortalEndUser and this has been given full Read Access to the Non-Process Related access in Knowledge Management.
However it didn't start working until I gave the role full Read access to the Process-Related Object > Article.
Don't know how I missed that, but thanks again!