My stats usually work, but today, going to the page on my WordPress installation returns:
You are not a member of this site.
If you have just installed the WordPress.com Stats plugin, this error probably appeared because the API key entered in the plugin belongs to a different WordPress.com user account.
The owner of the API key must visit this site's stats page to grant access.
This error seems to have come up years ago in threads here but the solutions seem to apply to much older versions.
I haven’t made any upgrades to plug-ins on my site since the stats last worked (which, I imagine, was a couple of days ago).
The stats on WordPress.com are still functioning.