Try reinstalling or updating your VMware tools to fix the issue

Symptoms

1. You are using one of the Acronis Cyber Backup products on an ESX Host to back up virtual machines;
2. The backup of one or several of the virtual machines fails with an error message similar to one of the following:

    

An error occurred while saving the snapshot: Failed to quiesce the virtual machine. An error occurred while taking a snapshot: Failed to quiesce the virtual machine.

Creating a quiesced snapshot failed

An error occurred while quiescing the virtual machine. See the virtual machine's event log for details.

VMware failed to create a quiesced snapshot of the selected virtual machine. To back up the machine, power off the machine before the process or uninstall the Volume Shadow Copy Services Support feature of VMware Tools from the VM.

Protocol error from VMX

An error occurred while saving the snapshot: A required file was not found.

Cause

Please see the solution section of this article for specific causes and resolution.

Solution

1.1 VM snapshot should work correctly

Acronis software uses snapshots executed by the hypervisor while taking the virtual machine backup. It is necessary that these snapshots work correctly. Create a virtual machine snapshot to check the consistency :

1. Open the vSphere connection to the ESX host.
2. Right click on the machine > take snapshot > check the "Quiesce guest file system" 
3. Uncheck the "Snapshot the virtual machine's memory" and click OK.

  

If taking quiesced snapshot fails, investigate the issue with the virtual machine (contact VMware support if needed)

In most cases, reinstalling/updating VMware tools and rebooting the virtual machine will resolve the issue with quiesced snapshot. Please see the following articles:

• VMware Knowledge Base Article 1009073
• VMware Knowledge Base Article 5962168
• VMware Knowledge Base Article 1007696
• VMware Knowledge Base Article 1007346
• VMware Knowledge Base Article 80246

If quiesced snapshot is created successfully but backup with Acronis still fails with the same error message, contact Acronis Support with a reference to this article

1.2 Verify VSS status

If you backup a virtual machine with transactional applications installed (SQL, Exchange evt.), then:

• VMware Tools should be installed inside that guest OS.
• VSS service should not be disabled inside the guest OS.

In case the guest OS event logs contain errors or warnings for VSS operations during creation of the snapshot, troubleshoot the VSS issues inside the guest OS.

If a virtual machine snapshot fails due to incorrect guest file system quiescing (snapshot without option Quiesce quest file system is successful), for virtual machines with no transcational applications installed you can workaround the issue by uninstalling the VMware Tools for the guest OS (see More information below) or by disabling Volume Shadow Copy (VSS) for Virtual Machines backup option (for Acronis Cyber Protect Cloud or Acronis Cyber Backup 12 and above).

Keep in mind the VMware ESX(i) virtualization platform limitations:

• Agent for VMware backs up a fault tolerant machine only if fault tolerance was enabled in VMware vSphere 6.0 and later,
• VMware vSphere does not support snapshots of independent disks and Raw Device Mapping (RDM) disks in physical compatibility mode. To overcome these limitations install an agent into the guest OS.

1.3 If the error message is:

Creating a quiesced snapshot failed because the create snapshot operation exceeded the time limit for holding off I/O in the frozen virtual machine

Then see the following article for a solution: VMware Knowledge Base Article 1038963 

If ESX(i) version is 6.0, install this patch.

 The error may be caused by pre-backup command failure. A pre-backup command (pre-freeze script located in /usr/sbin/pre-freeze-script on a Linux machine or in a specific directory on a Windows machine) is a user-supplied script that should be executed before backup. Usually, these scripts are used to quiesce an application that does not have its own VSS provider (in Windows) to ensure a consistent state before backup. When such a script exits with a non-zero return code, this indicates that the script failed to perform its quiescing task and quiesced snapshot operation has to be cancelled.

 See also the following solutions:

• If VM freezes during snapshot creation, see A virtual machine can freeze under load when you take quiesced snapshots or use custom quiescing scripts;
• If VM runs a database, try giving it more processors/RAM. Allow VM to use more of the host/cluster resources.

More information

Acronis Cyber Backup 12.5: If the backed up VM does not run a database, you can disable quiescing of VMs via VSS by turning off Volume Shadow Copy for virtual machines (applies to Acronis Cyber Backup 12.5 only - there is no such option in 11.7 or lower). With quiescing disabled, the VM will be backed up in crash-consistent state.

For Acronis Cyber Backup 11.7 and older products, try disabling the VSS component of VMware Tools to solve the issue, but only if the backed up VM does not run a database:

Click to expand instructions for Acronis Cyber Backup 11.7 and older products

Uninstall the Volume Shadow Copy Services Support feature of VMware Tools from the virtual machine:

1. Select the virtual machine in question on ESX server and go to Inventory -> Virtual Machine -> Install/Upgrade VMware Tools:
2. 
3. This will launch the installation of VMware Tools on the virtual machine:

   

4. Choose Modify:

   

5. Uncheck the Volume Shadow Copy Services Support:

   

6. Complete the modification:

   

7. Reboot the virtual machine;
8. Create a backup of this machine which should succeed this time.