Has it ever worked on the purple stick? I feel like it may never have been officially supported. Very slow, old hardware.
Sling's Website say "LT and higher" https://help.sling.com/en/support/solutions/articles/33000218990-what-devices-support-the-sling-tv-app-
I suspect that's out of date info, and agree that the purple Stick (that's the one from 2014, isn't it?) is just too outdated for apps such as Sling TV. I wouldn't suggest running it on anything older than a Roku 3.