That have never happened before, it started happening some months ago (about when I started to use CloudFlare?). None errors appear in the Server Error Logs when this happens.
Yes, I do not have the newest version of XenForo. I don't want to upgrade just to get the newest version, still my XF 1.2 is a valid lisence. Anyways, I doubt very much this has something to do with the XenForo version. My XenForo version is 1.2.1, anyways.