iTunes 11.0.1 (22)
Mac OS X 10.6.8
218.83 GB available
I get this error occasionally since getting the iPhone5 in October. My last successful back up to thei Mac was yesterday. Tonight, no backup.
One time when this occurred I had trashed files, and after I emptied the Trash the sympton went away. Tonight I had files in the Trash and emptie the Trash before connecting my iPhone and now no backup allowed.
I have quit and relaunched iTunes, and unconnected and connected the iPhone tonight with no change in the symptom.
My next counter measure will be to reboot.