Last week I could not upload any XAP files to Windows Phone Dev Center. As soon as I would select the XAP to update or upload, I would get an error saying "Failed to Invoke: savePageData" like you see in the below screenshot. While it does seem like an issue on the DEV CENTER, it's actually due to Silverlight being messed up on your machine. To resolve this, uninstalled Silverlight and then reinstall it by visiting http://www.microsoft.com/getsilverlight/get-started/install/default.aspx and then clear your browser's cache. Doing these two steps should get it working again.