Signing out and back in should solve the problem.
Edit: Hmm, it says I don’t own the game if I click “Diablo II (2000)”, but works fine if I click the “General Discussion” subcategory. If I click “Diablo II (2000)” then click “General Discussion” on the top afterwards, it says I still can’t post… I guess there’s some bugs in the web form that need squashing.
Edit2: Seems to be resolved.