Just few days ago i posted about the issue here: https://forums.opera.com/topic/32741/opera-mini-proxy-in-automatic-data-saving
Long story short, the reason of all those issues, is simple: It's the Opera Mini Proxy
This means that the entire browsing experience gets slower simply because all the requests go through Opera's servers located around the world.
The IP addresses keep changing every hour making everything unstable and not reliable, that's why cookies are automatically deleted and you're logged out every time.
Also, it's not absolutely safe to login to gmail or your personal internet banking because everything is intercepted by these proxies that are saving all your data logs as well.
The solution is to turn off the data saving, but at this stage it would be better to uninstall the browser altogether and use another one.