Backup fails with Error 19

I have been backing up my MacBook Air for many months to an external drive on a Mac mini. My husband has also been doing the same from his Air. I recently started receiving the message "The backup disk image “C's Air.sparsebundle” could not be accessed (error 19)." My husband's Air continues to be backed up. Both sparcebundle "files" are on the same partition of the external hard drive. When I try to do a backup the "lock" file in the C's Air.sparcebundle directory is created.