I think I've resolved this using the advice listed here: https://ubuntuforums.org/showthread.php?t=1593587&p=10267762#post10267762

TL/DR: Changed MTU in the network settings from Automatic to 1492

I've enabled VPN in the Settings tab, and the VPN badge stays orange forever. It just says "Connecting..." and the Virtual location dropdown has "Optimal location" only. How do I fix/debug that?

It's Opera 45.0.2552.812 on Ubuntu 14.04.4 LTS (x86_64, Unity)

Related or not, I cannot log into Opera forums site from that computer. It says "auth.opera.com didn't send any data". Other HTTPS sites seem to work