This solution applies to Yosemite Server Backup, version 8.0 and above.
When you see a 3028 ?Unknown Error?, this typically means the operating system reported an error that Yosemite Server Backup software did not recognize.
To find more information about the error, locate the yttrace files. By default, these are found in the Log folder under the directory where Yosemite Server Backup is installed. From these logs you should be able to see the error that was generated by the operating system. You should be able do a web search on the error code being returned by the operating system and find out what could be causing the issue.
In some rare cases it has been seen that this error can post when trying to backup a mapped drive with Yosemite.
Disabling the "Native Data Streams" option under the Advanced options of the backup job has resolved this case.
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