Was
- UbuntuMATE hosting VirtualBox,
or
- was VirtualBox hosting UbuntuMATE ?
Also, if the first, was the VirtualBox hosting Windows/Mac and doing something ?
I offer this if you haven't done the lookup yet:
Also, don't know if you need to review this discussion:
And this one seems share your specific observations, and the feedback seems to suggest the problem is related to NFS, with the recommendation being to switch to iSCSI: