Settings => All Websites doesn't show websites for superuser (no replies)

Hallo everybody,

We are hosting a very large enterprise installation of Piwik and we just did a test update from 1.12 to 2.6.1. Everything works so far, except that the site Settings => All Websites doesn't show any websites although I'm logged in as super user:
[attachment 1922 screenshot.png]

I tried to debug this issue. The problem is that this request doesn't give any results:
/index.php?date=yesterday&fetchAliasUrls=true&format=JSON2&idSite=1&method=SitesManager.getSitesWithAdminAccess&module=API&period=day

These are the request details (from Chrome dev tools):
[attachment 1924 screenshot2.png]

I set a log statement to SitesManager\API.php in the method getSitesWithAdminAccess and it retrieves the correct sites (all of them because I'm superuser):

public function getSitesWithAdminAccess($fetchAliasUrls = false)
{
    $sitesId = $this->getSitesIdWithAdminAccess();
    $sites = $this->getSitesFromIds($sitesId);

    if ($fetchAliasUrls)
        foreach ($sites as &$site)
            $site['alias_urls'] = API::getInstance()->getSiteUrlsFromId($site['idsite']);
	// here, $sites is a huge array with more than 1000 sites
    return $sites;
}

However, the request doesn't get any JSON response ( Content-Length:0 ). The strange thing is that I can make this request if I set the format parameter to "XML" instead of "JSON2". It works when I just put it in the browser address bar, so it doesn't even care if it's a GET request instead of a POST. As soon as I set format=JSON2, it doesn't work again.

I think we really need help here!

Regards,
Felix