I am running wordpress 3.5.2 with a Piwik version 1.12 install. It is running version 0.9.9.6 of wp-piwik. I get the "An error occured: Piwik did not answer. Please check your entered Piwik URL." message on this site.
I have one site (a wordpress 3.5 install, I haven't updated it yet) running version 0.9.7 of the plugin, and it works great. I have tried copying the URL and my API key from this site to the one that isn't working to make sure the settings are right, but it still gives me the error.
If I run the test script under the support tab (on the problem site), both tests return "HTTP/1.1 200 OK", so the plugin seems to be able to connect to my Piwik server, but it doesn't think that it is working. The html returned also looks correct, with the first test returning the details on all my sites in piwik and the second test returning the correct site ID.
I have tried the 0.9.7 version of wp-piwik on the 3.5.2 site that is giving me trouble and it works great. So I assume it is some sort of compatibility problem between WP 3.5.2 and wp-piwik 0.9.9.6. Infact, none of the plugins after 0.9.9.1 work for me (some make nothing but blank pages show, others have activation errors and the later ones [0.9.9.4+] give the same error as I am having with the latest version).
Any suggestions for things to try to solve this would be really appreciated!