I have tried setting the WP-Piwik 0.9.9.6 plugin up with the latest WordPress for a couple of sites and although the "WP-Piwik Settings" "Run test script" returns the correct value for the Site ID in the XML the "Get site configuration details" always returns "1" and the tracking code can't be fetched, the "Tracking code preview":
<!-- WP-Piwik ERROR: Tracking code not availbale -->
(Also note the spelling mistake above, s/availbale/available/).
Trying to view stats results in:
Piwik error: You can't access this resource as it requires an 'view' access for the website id = 1.
It might, or might not, be significant that the Piwik site is only available via HTTPS.
If there is anymore information needed to help debug this problem please let me know.