Check your opera's 28.8.1750.48 folder. You'll find it where you installed opera. Open this folder and you should see opera_autoupdate.exe if it is missing or renamed because someone deleted or renamed because he/she didn't want the automatic update to kick in. That is the error you will see if a change was made to it. Myself I just add .old at the end of .exe I like to check manually for an update. When I am ready I delete the .old and go to opera menu and select "About Opera". If there is an update, it will let me know and download the update. Doing it this way, I control when I want to update opera.
Edit: Some other things that can prevent autoupdate access: Seen here: https://forums.opera.com/topic/645/how-to-disable-opera-auto-update/9