Have you tried flushing the caches?

Paths to classes are cached, so this should help.

Te recomiendo que antes de hacer un cambio de esa magnitud, lo haga en un sitio de prueba. Elgg se puede actualizar correctamente sin ningun inconveniente. Los problemas se podrían dar con el tema o los plugins

Isn't it the php7 mysqli extension?

@cim

Not really. Elgg 1.8x, elgg 1.9x and even some versions of 2.x are not compatible with PHP 7 because of that "missing mysql extension" error. 

I wanted the search result to return only a list of object entities not user. For example. in the blog page, I want to list blog posts of a specific user by using a username as the search query. 

Is the username a value in metadata of your subtype?

yes, it is.