It is caused by the following error:
Undefined index: post_type in /var/www/wp-content/plugins/search-by-sku-for-woocommerce/woocommerce-searchbysku.php on line 28
This should be fixable by checking the post type beforehand with a an isset() function or something similar. So far, I haven’t had luck fixing it, but if anyone else knows how to fix this error quick, please chime in.
For now, I have reverted to using version 0.5.2, which you can grab from git-hub here: https://github.com/wp-plugins/search-by-sku-for-woocommerce/tree/e5202e44fb2ed43b99e023be4c07a48a50a44255
I am using WP 4.0 and WC 2.2.7