I’m fairly sure I had the same problem as you and my fix was to edit line 372 in the file wp-content/plugins/wp_smf/wp_smf.php
from:
$smf_api = ABSPATH.'wp-content/plugins/WP_SMF/smf_api_1.1.php';
to
$smf_api = ABSPATH.'wp-content/plugins/wp_smf/smf_api_1.1.php';
Paths are often case-sensitive!