Trying a simple export of a vm yields an inexplicable error, please see below. Host is ubuntu 14.04, virtual box is version 4.3.20, guest is win7, internal disk format is vmdk, and of course the guest is shut down. I am running as me (not root), there is plenty of free disk space everywhere, the current working directory is my home directory, I have read and write permission for my home directory, for the "VirtualBox VMs" directory, and for all the files in that directory. (I made absolutely sure by doing chmod -R og+r "VirtualBox VMs".)
% vboxmanage export win7x64 -o win7x64_new.ova
0%...
Progress state: E_ACCESSDENIED
VBoxManage: error: Appliance write failed
VBoxManage: error: The object is not ready
VBoxManage: error: Details: code E_ACCESSDENIED (0x80070005), component Medium, interface IMedium
VBoxManage: error: Context: "int handleExportAppliance(HandlerArg*)" at line 1121 of file VBoxManageAppliance.cpp
Google turns up so few similar issues that I'm confident I must be doing something wrong, something has gone sick, etc. Please tell me, there's no Virtualbox service to restart on ubuntu, right?