I have the same error as you. In my case it turned out to be a dateless time machine local snapshot.
To check for it, open a Terminal and list local time machine snapshot by running the
command «tmutil listlocalsnapshots /»
It will show entries like «com.apple.TimeMachine.YYYY-MM-DD-HHMMSS»
If you see dateless snapshot, delete it use «sudo tmutil deletelocalsnapshots YYYY-MM-DD-HHMMSS»
with the actual date of the dateless entry. You will need to enter your user password.
# tmutil listlocalsnapshots /
com.apple.TimeMachine.2017-10-04-014927 (dataless)
com.apple.TimeMachine.2017-10-04-024750
com.apple.TimeMachine.2017-10-04-024752
com.apple.TimeMachine.2017-10-04-024835
com.apple.TimeMachine.2017-10-04-024852
# sudo tmutil deletelocalsnapshots 2017-10-04-014927
Password:
Deleted local snapshot ‘2017-10-04-014927′
# tmutil listlocalsnapshots /
com.apple.TimeMachine.2017-10-04-024750
com.apple.TimeMachine.2017-10-04-024752
com.apple.TimeMachine.2017-10-04-024835
com.apple.TimeMachine.2017-10-04-024852