Using 5.1 here and having the same problem with linux. Doesn't seem to matter if the host is using DHCP or not. I'm also really not sure how this would actually matter, as I have the system powered off when trying to export it.
Edit: I actually switched the system to DHCP just to try and see if things would work, boot it and made sure it got an address then shut it down for export. I still get the same issue where the export starts and some where around 3 GB, the whole operation slows to a trickle and eventually I get the error: Failed to export virtual machine: The operation has timed out.