Unknown traffic from 72.21.211.174

I just discovered that my Opera was constantly receiving data from IP 72.21.211.174 (which tracks to an Amazon server) - I'm talking about 1-2 GB per day! I blocked this IP address now in the firewall, but has anyone an idea why this was happening? It caused us to go way over the limit of our Internet subscription

That Amazon.com IP is considered by a variety of Internet IP rating services to constitute a "high-risk" hosting service. Which means there have been observed to be a higher-than-normal number of security "threats" that have operated via their server, as well as ordinary web sites and services. Usually this is because a hosting provider is less than rigorous in vetting its clients who rent its server space, or is less than secure in preventing its hosted sites from being maliciously hacked. A concern that needs to be addressed by you is whether there is something malicious now on your system that is "phoning home" to that IP block for whatever reason, perhaps using your computer as a kind of relay or tracking certain activities on your system.

Do you have any kind of Opera extension that might have a legitimate reason to communicate with that IP? Otherwise, I'd certainly clear Opera's caches of anything stored there, then run some full-drive security scans of your system (AV, Malwarebytes, etc) to make sure it's clean. There has to be something on your computer or operating in/under Opera that is triggering communications with that IP link.