I have personally seen this error in R9.1, but it may also be in earlier versions.
When backing up Server Edition configuration, it will reach 20% and then display the following error:
Backup failed: Wait inventory pushed stage failed
In my experience, this was caused when trying to backup to the Server Edition itself and there was not enough space on the hard drive to backup to.
Increasing hard drive space in the server or pointing the backup to a remote server instead resolved the issue.