How can I fix "MTM fs Mount server failed to start because of error 15"?
I had the same problem and was due to errors in the localhost's IPv4 address in /etc/hosts. The daemon of mtmfs listening on localhost.

Check that this is the first line in /etc/hosts:

127.0.0.1       localhost


Also you can test: (depends on the firewall options)

$ ping localhost


Then turn off local Time Machine snapshots and trigger automatic cleanup of accumulated local snapshot data :

$ sudo tmutil disablelocal


Reboot the mac, and turn on local Time Machine snapshots:

$ sudo tmutil enablelocal


And it works without errors.

[system.log] mtmfs[40]: mount succeeded for /Volumes/MobileBackups