NBU Unable to remove VM snapshot SYM_VMC_SOAP_ERROR

Hello,

After upgrading NBU master from version 7.0.1.4 to latest version 7.7.2, snapshot created by NBU for VMs Backup (called "NBU_SNAPSHOT Master xxxxxx") are not removed automatically and I need to delete them manually.

Master server and vCenter server OS: Windows 2008 R2

VMware vCenter Server is v. 4.1.0 build-925676.

NBU Client on vCenter Server is v 7.7.2

NBU Job log report this error:

04/14/2016 17:20:28 - Info bpbrm (pid=8088) INF - vmwareLogger: removeVirtualMachineSnapshot: <InvalidRequestFault xmlns="urn:vim25" xsi:type="InvalidRequest"></InvalidRequestFault>
04/14/2016 17:20:28 - Info bpbrm (pid=8088) INF - vmwareLogger: removeVirtualMachineSnapshot: SYM_VMC_ERROR:  SOAP_ERROR
04/14/2016 17:20:29 - Critical bpbrm (pid=8088) from client <removed>: do_thaw: Unable to remove virtual machine snapshot, status=6 (SYM_VMC_SOAP_ERROR). Error Details: [].

I've already read a lot of similar post in NBU forum and I've applied different workarounds but with no luck.

attached to this post you will find the bpfis log fot the Backup Job.

Thanks for your help

Regards

F.