VEEAM Failed to flush change tracking data. Using full scan incremental run.

Changed block tracking information is no longer being read, causing entire disks to be read instead of just the changed blocks.
This is a very generic error, so it is quite difficult to say what has happened.

Hyper-v Solution:

Run “sc stop veeamfct” and “sc start veeamfct” on the Hyper-V hosts affected to restart the Veeam Change Tracking service, and then see if the errors are resolved afterwards.

VEEAM KB1881

OR

When you run VEEAM V8, you must install patch 1. After this update the Flush change tracking data errors are gone.
This patch can be downloaded from the here:
VEEAM KB1982

General access denied error (0x80070005)

This issue occurs when the VM Security permissions on the VHDx are gone. Every VM has a unique ID. This unique ID is added to the VHDx security permissions for accessing / locking files.
There are 2 possible solutions to fix this.

Solution 1:

Detach the VHDx with the “remove” remove button in Hyper-v or Failover cluster manager. Just make sure you don’t remove the actual file!

Attach the VHDx again.
Now you can start the VM.

Solution 2:
  1. In this example is the virtual machine ID 684B169F-DB4A-493F-B34A-762E205AB1E4
  2. Go to the node wich host this VM
  3. Open an elevated command prompt
  4. icacls <path of vhd> /grant "NT VIRTUAL MACHINE\<Virtual machine ID>":(F)
  5. Start the virtual machine