Goto the URL opera://settings/privacy, disable "Display promotional notifications", restart Opera and see if that helps.
Also, just for good measure, you might want to reset the sidebar settings. Goto the URL opera://about and take note of the "profile" directory. Then, close down Opera and edit the "Preferences" file with JSONedit. Under root/ui/browser, right-click the "sidebar" node, delete it and save your changes. Then, when Opera starts, go ahead and click the 3 dots at the bottom of the sidebar. Then, disable all the sidebar panels you want, hide the sidebar and restart Opera. Then, see how things go. If Opera adds a new feature or if Opera updates, you might get bugged about some of the panels again though.
