If you are using child themes and you have header.php in it then upgrade Divi it might cause the search form to stop working.

The simple fix is to remove your header.php from the Child theme e.g

rm -f blog/wp-content/themes/Divi-child/header.php

If you have header.php customisations you want to keep then you need to copy the new header accross and merge the changes:

# take a backup
mv blog/wp-content/themes/Divi-child/header.php ~/header.php.old

# copy the new header.php to the child theme 
cp blog/wp-content/themes/Divi/header.php blog/wp-content/themes/Divi-child/

# edit to taste

vi blog/wp-content/themes/Divi-child/header.php