I have the same issue. Android devices and Chrome browser cast perfectly but Opera doesn't -- I get "No devices found".
When I upgraded one of my PCs to Opera v 62.0.3331.72 from an earlier version (not sure which) it connected to Chromecast once the first time, but since then I get "No devices found". (I can ping the Chromecast dongle and the TV to which it is connected from both PCs without any problems.)