tar xvf myfile.ova -C /home/USER
Above command extracts three file types:
-vmdk
-ovf
-mf
Then, from virtualbox create a new virtual machine (important: if machine exported was 32bits, new machine should be 32bits, and if exported was 64bits then new machine should be 64bits). Finally attach the existing vmdk file instead of creating a new one and that's all. boot up the virtual machine and it works in my case.
Anyway, I must say that despite many people say virtualbox is the best, since a lot of time ago, virtualbox always has problems when performing export/import and I think Oracle should work hard on it to try to solve it because If not many people maybe will change to the competitor: vmware. I am a bit tired, import/export with virtualbox is really a sh....