I have a lot of copy-pastes issues since i use Windows 10, and a lot of them are using Opera but not only. So maybe it's not a issue from Opera itself ?
The only way to make a copy-paste works 100% of the time is to copy by select the text and press Ctrl+C (or x for cut) and to paste with Ctrl+V. It will works everywhere but not to/from a virtual machine ...probably.
If you are not using Windows, i don't know how to help. I hope you are using windows and it help you (or others).