I have same problem. Here is my error log
[06-Jan-2017 10:47:16 UTC] PHP Fatal error: Uncaught Error: [] operator not supported for strings in /Applications/MAMP/htdocs/wordpress/wp-content/plugins/bbpress/includes/forums/functions.php:1800
Stack trace:
#0 /Applications/MAMP/htdocs/wordpress/wp-includes/class-wp-hook.php(298): bbp_pre_get_posts_normalize_forum_visibility(Object(WP_Query))
#1 /Applications/MAMP/htdocs/wordpress/wp-includes/class-wp-hook.php(323): WP_Hook->apply_filters('', Array)
#2 /Applications/MAMP/htdocs/wordpress/wp-includes/plugin.php(515): WP_Hook->do_action(Array)
#3 /Applications/MAMP/htdocs/wordpress/wp-includes/class-wp-query.php(1681): do_action_ref_array('pre_get_posts', Array)
#4 /Applications/MAMP/htdocs/wordpress/wp-includes/class-wp-query.php(3238): WP_Query->get_posts()
#5 /Applications/MAMP/htdocs/wordpress/wp-includes/class-wp.php(617): WP_Query->query(Array)
#6 /Applications/MAMP/htdocs/wordpress/wp-includes/class-wp.php(735): WP->query_posts()
#7 /Applications/MAMP/htdocs/wordpress/wp-includes/functions.php(955): WP->main(Array)
#8 /Applications/MAMP/htdoc in /Applications/MAMP/htdocs/wordpress/wp-content/plugins/bbpress/includes/forums/functions.php on line 1800
