Yes, I do have add-on extensions installed, but they have nothing to do with this issue.
Well, you are the one saying that the problem is with an extension by posting your topic in 'Opera Addons' section/area of the forum.
Does anyone know what are the search engine links to twitter, youtube and startpage specifically for Opera?
Why not just right click on the search field on those pages and use the 'create search engine' feature?