I saw this myself some time ago where one of my Rokus would allow me to login or subscribe while the other one would only allow me to subscribe. I didn't really look into it, but I thought it was odd. Make sure your software is up to date - Settings->System->System update->Check now.
If that doesn't help, try removing the channel from the Home Screen, reboot/restart (Settings->System->System restart), and then add the channel again.
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