Magic words: open-vm-tools, VMware, Vagrant, shared folders.
I had issues w/ custom Vagrant boxes when moving from proprietary VMware tools to open-source open-vm-tools (found in Debian & other distros).
Exact error message was
Stdout: Error: There was an error mounting the Shared Folders file system inside the guest operating system
Reason for the error was missing /mnt/hgfs folder in VM image that I had created with packer.io. Fixed that by creating missing path in Packer provisioning script. See the commit.
And oh, for my code, see github.com/ypcs/packer-templates.