Not sure if this is the same issue others are having, but apparently the 404 error that I was getting was because of a permissions issue. I first noticed there were messages in my cPanel error log that mention insufficient permissions. My hosting company had blocked access wp-cron.php because:
"Your script /home/myusername/public_html/mydomainname.com/wp-cron.php was causing extremely high processing on MYSHAREDSERVERNAME :
As such, the script has been renamed as /home/myusername/public_html/mydomainname.com/wp-cron.php_contact.support. This means that the script is no longer accessible and cannot use up the resources on the server. Please let us know what changes you will be making to correct this issue. If you are using plugins for Word Press or Joomla, we ask that you check them for available updates."
So, I'm still thinking this is a backwpup issue.
Anyone have any thoughts on narrowing it down or coming up with a solution other than trying a different backup plugin?