[WordPress 3.6] Search trouble (frontend and backend)

I have just updated to wordpress 3.6 and my theme seems to be ignoring my searchform.php file. I’ve use following code https://www.ads-software.com/support/topic/36-upgrade-searchformphp-not-working?replies=9#post-4487445 in function.php, replacing get_search_form($echo) in template file with get_search_form().
So, in frontend search works.
In backend no: if I try to search a post, the page become blank and empty (page is not loaded).
I tried to deactivate cache plugin, nothing change.
Someone can help me to resolve?

tnx