I've found another temporary solution. Go to Menu->Appearance in the "Buttons" tab choose "Search" category, drag and drop "Startpage Search Engine" field on the Address Bar next to the address field. Now you have a startpage search field that won't be overwritten next time you start opera. It's not like searching directly in the address field, but it's something.

I've found a temporary solution for myself. I just removed the search field from the address bar. I set the "s" as a keyword for "startpage search" and now I search directly from the address bar with "s" in front of the keywords (as in the old times ) It's very annoying though! The same as Opera on Android where you can't change the default search engine on anything else than google.