I recently restored a Groupwise 7.0.3HP1 post office from a daily differential backup. After logging into a newly restored mailbox, I received a Path not Found  error when attempting to create an archive from within the Groupwise client.
The reason for the 8209 error ended up being that the daily differential backup did not capture the ngwguard.dc file located in the root of the post office directory. Once I copied this file from the live post office to the restore location, I was able the create the archive. I also could have copied the file from the po directory in the Groupwise software distribution directory.
For additional details, see Novell TID 10052390