Solution! Found here.
(Note the most recent OSX supplemental update did not fix this issue)
To prove I had the problem my Plugins page on Development was empty, even though the folder was full. If I SSH’d into the Vagrant box and tried
ls in the plugins folder it gave the error
reading directory '.': Too many levels of symbolic links
Find this line:
config.vm.synced_folder local_site_path(site), nfs_path(name), type: 'nfs'
Add to the end:
, mount_options: [ 'nolock', 'vers=3', 'tcp', 'fsc', 'rw', 'noatime']
Then Vagrant halt, Vagrant up. Continue about your business.