Your case may be different... I had previously converted some hyper-v servers on a hyper-v host using the standalone converter... in my case I had to uninstall the agent from my host... run the standalone converter (version6) again... it THEN determined oh the that host has no agent let's reinstall it... once it reinstalled it's "own" agent we were good!
Worth a shot if you haven't done it... maybe uninstall the agent on the host machine you're making the call to to convert?