Other possibilities include a background page running in a speed dial or tab that has a website-code callout to the adnxs server, or an adware infection in your Opera installation. Fortunately (somewhat), adnxs is a major 'legitimate' ad server and not inherently malware of itself.
Most likely the website-code callout you mention. Probably also on one of my fav blogs. Needed to close that one tab and everything was good... I have been able to replicate the problem since so I am fairly certain now.
@leocg said in Mysterious Uploads of Several GBs through Opera:
How did you find out that it comes from Opera? And what about those addresses?