Try the following - under the file menu, select "Back Up Database..."; a sheet should appear. In that sheet, pick the folder you'd like the archive to be placed in.
Once OmniFocus completes the backup process, try archiving again - does it work?
If so, please send an email to
OmniFocus@omnigroup.com asking the support ninjas to add said email to the "Make the archive feature use the automatic backup location preference instead of the manual backup location preference" item in our development database.