I was also getting the "Error 404. The requested page (/scheduletask/runtask) was not found" errors in the log. I was also getting some 404 errors in the log that had no other details. Then I made the following changes:
* Added https://www to the Store URL in Admin\Configuration\Stores. This alone didn't solve the problem but was part of the issue.
* Made the Customer "[email protected]" an Administrator in Customer Roles by going to [mywebsite.com]/Admin/Customer/Edit/8 (note that "[email protected]" didn't show up in the list of customers when I went back to see what I had changed so I had to click the Edit button to open another customer and then changed id number in the address bar to 8). "[email protected]" does not have a password nor a record in the EmmailAccount db table so it is a special user.
* Made sure that "Specified store URL matches this store URL" was displayed in Admin\System\Warnings.
Both the http and https versions of the store url worked as far as having "Specified store URL matches this store URL" displayed but it needed to be https since my site forces https.
Once I made those changes, all scheduled tasks ran including the Contact Us email messages.
I hope this info helps.