This solution by Xs4all worked for me with the 105-3015 error.
I had the same Error Code: 105-3015 message even though I had the update folder, in the past I did remove this folder to stop updates and added the update folder back ages ago with the correct FOLDER PERMISSIONS.
This is what I did to fix, hopefully it will work for you and others who have the same Error Code message.
- Boot into CFW and launch FTPiiU Everywhere
- User you preferred FTP client, I'm using FileZilla.
- Locate the update folder at the following location: /storage_mlc/sys
- Right Click on the update folder and select File permissions..., take note of the Numeric Value: 755.
- If you have deleted the update folder in the past and added it back again, chances are that you may have not changed the Numeric Value to 755, but in my case it was already set to 755.
- Delete the update folder.
- Re create the update folder.
- Right Click on the update folder and select File permissions..., and change the Numeric Value to 755 then click OK to save.
- Now reboot your WiiU console and try to do a system update again, this time around, it should start updating.