@mr_absinthe
OK, glad you could finally test with PEAR and found that works, helps me to track the issue. Not sure the link you gave is of any relevence, as you found, once PEAR is properly set up ,it works.
Try this change in mail_backups.php
replace (98):
$data = @fread($fp,filesize(DIR_FS_BACKUP . $filename));
with:
$data = '';
while (!feof($fp)) $data .= @fread($fp, 1024);