An Automation Engine task sometimes does not write files on the requested output path, when that path is on a share of a different computer in the network.
For example, the 'PDF export' task.
Solution
Add a user BGSYSTEM on the computer where the output folder is configured. Also give permissions to this user to write to the output folder.
Contact your IT-responsible if you don't know the password. The password MUST be the default password as specified by Esko.