After upgrading to iOS 7, the token used on MobileID doesn't work, and the Parallels Client is unable to authenticate the user.
iOS 7 changes the behavior of an API that MobileID calls to acquire the phone's device ID. As a security measure to prevent user's tokens from being copied or stolen, MobileID locks the user's token to the user's phone device by encrypting the token data with the device's hardware ID. Unfortunately, iOS unexpectedly changes the behaviour of the API that returns the device's hardware ID, which prevents MobileID from correctly decrypting the user's token data. Effectively, iOS 7 corrupts the existing token stored in the phone by MobileID.
Unfortunately, there is no way that MobileID can automatically recover the corrupted token, as there is no back door built into MobileID. Users who upgrade to iOS 7 will have to remove the corrupted token and reinstall it. Please note that the MobileID app itself does not need to be removed and reinstalled; it is the user's token that has to be removed and reinstalled.