There was a major problem discovered by VMware which affects Changed Block Tracking (CBT) functionality on vSphere 6. It has been discovered and documented by VMware in KB2136854.
This problem affects all VMs running on ESXi 6.0: the incremental backups of these VMs may not be recoverable.
VMware has released a patch containing the fix for this issue. Please download and install it.
As workaround, you can either downgrade ESXi hosts to 5.5 version or disable CBT option in backup settings of Acronis Backup (Advanced) for VMware. Note that taking incremental backups will take longer with CBT option disabled, however the size of them will still be "incremental".