I don't know if this could help, but I came back to https://www.codecademy.com/learn/learn-java and tried to go to an exersice, that I haven't unlocked yet.
After that, this thingy bumped to my display:
"Oops! It looks like you can't access this exercise yet! Click below to resume where you left off."
I clicked resume, and after it everything was fine.