I had problems with League Pass too. I wanted to watch retro games so I signed up for League Pass. Every time I logged in, the text "Log in" blabla, "Try it for free" etc. kept appearing...although I was logged in I kept gettings ads for League Pass...clicking on these buttons again did not work or gave me error messages...
In short, log in once, then do NOT click on "sign in", "try it" etc. although they are displayed, but click on "Watch" and then "Featured":
https://www.nba.com/watch/featuredScroll down past a lot of entries to find Classics...
Other interesting videos are in the "NBA TV" section:
https://www.nba.com/watch/nba-tv