The Error Code 13 is the standard Linux file error code of EACCESS: meaning there is an error on access permissions. The copy offload tool is running as the configured "OpenStack" user and said process identification does not seem to have access rights to your backend storage volume. Changing file access permissions on the backend storage volume will correct this issue.
Let's say that the configured OpenStack user is "glenng". When the copy offload tool runs, it is trying to access the storage as user "glenng". One would need to look at the backend volume configuration to determine if "glenng" will have access rights to the volume. If you see:
>-rwx------ root root volume-as7836485-4846733
then it is clear that "glenng" would not be able to access the volume as in this example, it is owned by root and no others have access rights. Changing the rights to allow "glenng" access will solve this problem.