Have you or a dev edited that file? If not, re-upload it from the XenForo package. Probably worth checking / posting the contents here to see what has changed.
What is strange is that I did the update from 2.012 to 2.1 a few nights ago (Tuesday) and only now got the error. However, I don't know how frequently the file check is done. So maybe this was the first check since the upgrade?