The same thing happened to me. I copied over my Vagrantfile from another project and forgot to change the NFS settings. This caused the current VM to mount the parent folder of the VM that I copied the original Vagrantfile from. So I did this:
vagrant halt
vagrant up
Still got the same error. So I tracked down my exports file located at /etc/exports which looks like this:
# VAGRANT-BEGIN: 501 64e10d4d-342e-4f55-b69a-97edda35742f
"/Users/[username]/vagrant" 192.168.11.12 -alldirs -mapall=501:20
# VAGRANT-END: 501 64e10d4d-342e-4f55-b69a-97edda35742f
# VAGRANT-BEGIN: 501 b3c6e7fe-95d4-48db-8876-c89d3e07af8d
"/Users/[username]/Sites/molitech-oms" 192.168.11.124 -alldirs -mapall=501:20
# VAGRANT-END: 501 b3c6e7fe-95d4-48db-8876-c89d3e07af8d
# VAGRANT-BEGIN: 501 3e2aabfd-ce63-441b-85a2-1b51e8beb863
"/Users/[username]/Sites/app" 192.168.11.125 -alldirs -mapall=501:20
# VAGRANT-END: 501 3e2aabfd-ce63-441b-85a2-1b51e8beb863
# VAGRANT-BEGIN: 501 ff92e353-9f6d-48ee-9d76-e29b33f1dd60
"/Users/[username]/Sites" 192.168.11.126 -alldirs -mapall=501:20
# VAGRANT-END: 501 ff92e353-9f6d-48ee-9d76-e29b33f1dd60
The last two paths are the same. So I deleted the last three lines and ran:
vagrant halt && vagrant up
Now everything seems to be working fine.
OSX 10.11.1
Vagrant 1.7.4