bbPress.org

Here is the code for that exact error notice. bbp_check_for_blacklist is checking the comment blacklist in Settings > Discussion. Since you said that you do not have anything in the blacklist do you have another plugin with a blacklist that could be hooking into the default comment blacklist??

/** Reply Blacklist *******************************************************/
	if ( ! bbp_check_for_blacklist( $anonymous_data, $reply_author, $reply_title, $reply_content ) ) {
		bbp_add_error( 'bbp_reply_blacklist', __( '<strong>ERROR</strong>: Your reply cannot be created at this time.', 'bbpress' ) );
	}