App cannot get user id of some users

I have a PHP app that runs in an iframe. If a user visits the tab, he is redirected to a login page via getLoginUrl() and asked to grant permisson to access the standard user data. The app then obtains the user's ID and offers a PDF for download. This works well for most users. However, for a few users, getUser() does not return the user 开发者_如何转开发id. Likewise, extracting it from the signed_request does not work as base64_decode() does not return valid data. For the users in question, the problem exists on different machines, in several browsers (Firefox, Chrome, Safari, IE, Opera) and operating systems (Mac OS X, Windows).

It seems that this behavior started a few days ago or maybe it is only triggered by some user profiles. Does anybody have an idea what could trigger it and is there a work-around? Apps settings, user profile settings?