Hi @YouTube please find out what the unknown error that occurs is because for months it has been showing that I'm not subscribed to channels that I subscribed to so when I try to press the subscribe button for those channels, this "unknown error" occurs
Appreciate the additional info. Have you tried clearing cache & cookies: https://goo.gl/dwzcyo ? Also, try uninstalling the app, restarting the device and reinstalling the app. Let us know how that goes!