While setting up a new Sage 50 2015 server I kept getting an error message “An unknown error occurred compressing the database files.” After further troubleshooting with Microsoft Process Monitor (https://technet.microsoft.com/en-us/library/bb896645.aspx) I found that it was trying to put a temporary file in a folder called “Backup” inside the company folder. This is probably a rare condition and only seen if you try to select a backup folder other than the default.
Simply create a folder called “Backup” in the company folder and try it again.
Thanks! This solved my issue also. Would be nice if Sage would own the problem and post the fix on sagecity.com