It was also after a migration from vBulletin to xenforoAny updates on this? I have the same issue here. After migrating from vBulletin to XenForo 2.2 last week, I submitted supernature-forum.de/sitepmap.php to the Google Webmaster tools.
Google says it succesfully read the sitemap index and found 0 URLs.
When I look into the details, I see the same message as above - www.supernature-forum.de/sitemap-1.xml could to be retrieved, same for -2 and -3.
I just tried this and it formatted as XML for me, which at least indicates that it's syntactically valid, though there really isn't any reason it shouldn't be. Indeed putting it through a sitemap validator immediately comes back with no issues.
I don't know how hard it is to implement, but maybe it would help some customers (like me ) if the number of URLS per sitemap file was adjustable.