After upgrading my Plesk install past 8.1.1, I encountered a problem with the builtin backup utility. When attempting to create a new backup (either locally or to an FTP repository), I would almost instantly be handed back the error:
Unable to create backup session: Specified file is not accessible
I googled around and found a couple of results, including a support forum that actually had the answer to my problem burried back on the second page.
For whatever reason, Plesk loses the ability to write to its temporary directory, where all backups are held until they are completed (even for FTP destinations). I was easily able to solve this problem by (as root):
chown -R psaadm:psaadm /var/lib/psa/dumps
Note that the original author of the suggestion I used said to chmod 777 the files, but this proved to be unnecessary. I saw that the parent directory was owned by psaadm, and it just made sense that the dumps directory would need to be as well.
In any case, it worked for me. Hope this helps someone…