Random Replication Failures

Hi Everyone

I have a problem with vRanger Pro and some replication jobs failing intermittently.
My setup is this:
Two ESX 4.1 servers connected to a vCenter machine (Windows 2008r2)
vRanger Pro 5.0.0.19238 running on the vCenter Machine
Two Linux and one Windows server VMs that get replicated every half hour using hybrid replication
Every few hours the jobs will fail with the error message "API Call failed with message: The object has already been deleted or has not been completely created"
The logs on the esx box shows the following messages (in sequence). This machine has 3 disks.
Create virtual machine snapshot - Completed
Remove Snapshot - Unable to access file <unspecified filename> since it is locked
Remove Snapshot - The object has already been deleted or has not been completely created
Remove Snapshot - The object has already been deleted or has not been completely created
Remove Snapshot - The object has already been deleted or has not been completely created
Remove Snapshot - Completed

First of all, does this compromise the integrity of the replicate vm? And how can I get the job to run reliably?
Many thanks,
Sam.