stebcom on "[Plugin: WP Super Cache] Fatal error trying using dynamic content"

I'm trying to use dynamic content, as explained in this post:
http://ocaoimh.ie/y/6j

I defined the cache tag

define( 'DYNAMIC_CACHE_TEST_TAG', 'my-placeholder-string' );

and put this markup in my template (I need the cart widget not to be cached)

<!--my-placeholder-string-->
<?php woocommerce_mini_cart(); ?>
<!-- woocommerce_mini_cart(); -->
<!--/my-placeholder-string-->

Now I get this error:
PHP Fatal error: Unknown: Cannot use output buffering in output buffering display handlers in Unknown on line 0

If I delete the DYNAMIC_CACHE_TEST_TAG, then the site works again.

I'm using PHP cache, with dynamic cache and lazy loading options enabled.

What am I doing wrong?

https://wordpress.org/plugins/wp-super-cache/