Need advice on error message when converting backup to VM

I'm just trialling ABR Virtual Edition and it's all a little new to me. I've set up a test where a Win2003 x64 server is being backed up to another server which is Win2008 and hosting Hyper-V. I've set a full backup to take place every 4 hours and a differential backup to take place every 2 hours. Each job has the Convert to VM option enabled.

When the jobs first ran all went well but since then the differential Convert to VM operation has failed each time with the error below. Can anyone tell me what this means, why it happens and how to resolve it? Thanks in advance. The error text is -

Command 'Converting backup to VM 'Acronis_DIFF_Mary.domain.office'' has failed.
Additional info:
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Error code: 61
Module: 309
LineInfo: 4a8728dc8a1c94f4
Fields: $module : service_process_vs
Message: Command 'Converting backup to VM 'Acronis_DIFF_Mary.domain.office.com'' has failed.
--------------------
Error code: 32795
Module: 114
LineInfo: fde974fd2ebcff5f
Fields: $module : disk_bundle_vs
Message: Failed to convert the backup to the virtual machine.
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Error code: 32855
Module: 114
LineInfo: fde974fd2ebcff0b
Fields: $module : disk_bundle_vs
Message: Failed to find any appropriate snapshot in the virtual machine.
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