OK, here is an excerpt from the provided log:

Starting vDisk upload...
Uploading: /COPY_TO_VM/nvram to null
Unable to import file COPY_FROM_VM-disk2.vmdk to null
java.lang.NullPointerException
        at de.sva.vco.plugin.ovatransfer.core.ImportWorker.run(ImportWorker.java:37)

So it seems that the import fails because the import destination is null / not set, which causes NullPointerException to be thrown in the Java code of the plug-in, followed by failure of the workflow.

Unfortunately, it is not possible to troubleshoot this further without having access to plug-in content (workflows/actions) and plug-in source code. This plug-in is not part of standard vRO plug-ins, so you need to contact plug-in authors.