In my case, the problem was caused by a lack of disk space (on backup destination).
There is "Can't do backup" error in BackInTime GUI and "writefd_unbuffered failed to write" rsync error in BackInTime log. No words about "disk space is exhausted".
In my case, the problem was caused by a lack of disk space (on backup destination).
There is "Can't do backup" error in BackInTime GUI and "writefd_unbuffered failed to write" rsync error in BackInTime log. No words about "disk space is exhausted".