@martant Are you running the VPN while trying to update?
These are my specs. I was able to get it to install. The reason you are not able to install is because Mavericks supported 32-bit. https://support.apple.com/en-us/HT208436#:~:text=Apple has been working with,no longer compatible with macOS.
The new Opera Software is most definitely 64-bit. As you are showing me here "Betriebssystem: Mac OS X 10.9.5 64-bit"
Also the fact you are getting an "error occurred proofing updates" that means there is something wrong with your Opera Application. Have you tried uninstalling it? Have you tried installing earlier versions of Opera instead of V 49.0? Like I said I was on 30 so try that. You should have no issue with it updating automatically since you can't even get it to do that manually.