Confirm the issue on Ubuntu.

I can try to load a page dozens of times with the same error. Sometimes it finally loads.

Opera Turbo is disabled.
Same behavior in private mode, with no extensions loaded. Though, I disabled all network-related extensions in the normal mode too.
Chromium and Firefox loads the same URLs at the same time without any problems.

Opera 40.0.2308.62
Ubuntu 14.04.5 LTS