According to the VMware Converter 5.5.2 Release Notes, Acronis Backup & Recovery 11.5 is not explicitly supported for conversion:
Third-party backup images and virtual machines
Acronis True Image Echo 9.1 and 9.5, and Acronis True Image Home 10 and 11 (.tib)
I know the image format ought to be the same, but I'm not sure there might be some minor changes in the way images are handled.
Error : Unable to contact the specified host...
Have you installed converter on a separate system and is this the error when you try to connect to the physical source machine? If a firewall blocks that incoming traffic, then you can just install Converter directly onto the physical source host as well. This way you only need the outgoing connections (which are often completely open) to vCenter and the ESXi hosts (TCP/443, TCP/902):