Hi
Most likely you are using YooTheme Pro/WidgetKit, which has a bug causing this. YooTheme has been informed about 3 weeks ago, they announced a fixed version and we're all waiting for it.
We did find the bug and detailed an applicable fix, so if you can change a line or 2 in WidgetKit until they released a fixed version, all details are on this paragraph of the 4SEO troubleshooting documentation.
If you don't use a YooTheme template and don't have WidgetKit at all, then there are other possible reasons, nearly all of them being a firewall or security system on the server used.
Best regards
Yannick Gaultier
weeblr.com / @weeblr