Clash with Envira & Soliloquy

Unfortunately these kind of issues are very hard to debug. There might be a conflict in the way the buttons are added but it cannot tell you what is the cause of it.

You can try to find where the buttons for the slider are being inserted and compare it with how this is done in the Custom Post Widget:

// Only add content_block icon above posts and pages
function cpw_add_content_block_button() {
	global $current_screen;
	if( 'content_block' != $current_screen -> post_type ) {
		add_filter( 'media_buttons_context', 'add_content_block_icon' );
		add_action( 'admin_footer', 'add_content_block_popup' );
	}
}
add_action( 'admin_head', 'cpw_add_content_block_button' );