When MobileTrans fails to back up iPhone WhatsApp messages, please use this way to transfer WhatsApp in MobileTrans-WhatsApp Transfer.
1. Back up your iPhone to this computer by iTunes. ( PS: Please don't check "Encrypt iPhone backup" in iTunes. )
2. Choose the feature of Restore WhatsApp messages to your Android device.
3. Select the iTunes backup in the list you just made and click on the Next button to restore WhatsApp messages to the destination phone.