The following error appears when trying to backup a domain:
<message id="12495ac2-9d13-4862-89d0-5eded4872d80" severity="warning" code="msgtext">
<description>Failed to pack files backup_domain.tld_user-data_1407021503 in /var/lib/psa/dumps/domains/domain.tld</description>
<message id="57478965-54fc-4456-857d-4147f88a50a8" severity="warning" code="msgtext">
<description>For security reason backup is performed on behalf of subscription system user. System user has no read access to httpdocs/apc.php file.</description>
Subscription system user does not have access to the mentioned in error file/directory.
Check permissions for FTP user/subscription user for mentioned in error file/directory. Make sure that it has at least read permissions.