I can't test against your app id because it's not public. Are you trying one of the examples from the project?
If you want to use v1.8, I think you need to use the steam_api.dll that's in the examples' folders since it was built on SDK 1.43. Newer version should work, but older ones won't because of missing interfaces in them, etc. Any particular reason you are using an older steam_api.dll?
Have you tried an older version of the plugin? 1.7 was built against SDK 1.42.
EDIT: Be sure that you're logged into Steam with an account that has access to the game in development.
EDIT 2: I'm pretty sure you're using the leaderboard example.
Open it up and change this line:
AddStatus("Error loading user stats.")
to:
AddStatus("Error loading user stats. Result: " + str(Steam.GetUserStatsReceivedResult()))
What is the result code it gives? 2?
From what I read
here, iRequestUserStats will report failure if there are no user stats. I'm not sure why this would happen, but then your user probably don't have any stats, achievements, or leaderboard entry for the game yet. Are you about to FindLeaderboard() your leaderboard name?