I had the same issue, since I had upgraded from Ubuntu 16.04 to 18.04, but changing the nfs version( in the Vagrantfile) didn’t solve it …
Checking the status of my nfs server (sudo systemctl status nfs-server.service) I discovered that I had a line from an older trellis site creating problems
exportfs: Failed to stat /home/ben/Code/mytestsite.test: No such file or directory
So all I needed to do was to clean the /etc/export file of the culprit lines pointing to that non existing directory
sudo vi /etc/export