Hi @icearity Opera Extensions are automatically disabled in Private window but you can enable it. Hover to the left, click on sidebar options > ... you will see the option to enable extension right there.
For that, you have to Backup | Reset.
"Off Topic Tip"
Follow the Signature's Backup | Reset link.
Take the opportunity to start a Backup plan and even create a Template Profile.
SyncBack | Synkron helps.
Windows 7 (x64)
Opera Test profile | Opera Backup | Reset