I'm having the exact same problem, signed up for the trial with my credit card on a PC, then cancelled to prevent it auto renewing. My account was immediately changed back to a basic account, contrary to what is clearly written in the help center.
Such false advertising is disappointing to say the least, and I can't believe this hasn't been fixed yet. Hopefully Dropbox will resolve this issue in the near future, but for now I think I'll find another file sharing platform - there's plenty out there...