Done that with the 25 number issue. After removing it, it still did not work to the search result.

Does anyone know what are the search engine links to twitter, youtube and startpage specifically for Opera?

Yes, I do have add-on extensions installed, but they have nothing to do with this issue.

Well I did add those search engines in the settings menu, but the problem is any search words in those sites don't lead to the results page(s) such as:
twitter.com
youtube.com
startpage.com
...they just lead to home sites instead.

For YouTube it's: http://www.youtube.com/results?search_query=%25s&search_type=&aq=f
But it still didn't work regardless of which word I typed.

I use Windows 7 32-bit OS and Opera version 41.0.2353.56.