Hm… I currently have no idea how to reproduce this error without changing the form’s target URL.
Let’s try this:
1. Open network admin -> settings -> WP-Piwik
2. Choose tracking tab
3. Post the current site’s URL here (URL 1)
4. Click at “Save settings”
5. Post the current site’s (= error site) URL here (URL 2)
And one more test:
1. Open network admin -> settings -> WP-Piwik again
2. Choose support tab
3. Click at “Get site configuration details”
4. Do you get an error site, too?
5. Post the current site’s URL here (URL 3)
Correct behaviour would be:
– No error site (of course)
– URL 1: /wp-admin/network/settings.php?page=wp-piwik/wp-piwik.php&tab=tracking
– URL 2: /wp-admin/network/settings.php?page=wp-piwik/wp-piwik.php&tab=tracking
– URL 3: /wp-admin/network/settings.php?page=wp-piwik/wp-piwik.php&tab=sitebrowser
One more question: Are you using any plugin changing permissions, admin page URLs, or something similar?
I’ll answer your mail later this day.