So, if you were to now remove the above code, your archive page will load as expected?
I think there is something fishy going on in your theme in regards to the wp posts loop.
Perhaps you have a pre_get_posts filter or one of the many other filters in WP / ACF which is modifying the $query.
It is very easy to accidentally break the global $query in WP and I believe this is what is happening to you.
Maybe in your code, you are hooking in somewhere and performing a new WP_Query object? If so, make sure you are resetting the query afterwards.
It may be beneficial to create a new WP install on your local server and just install ACF. Create the same field and edit the twenty12 theme to use this $posts loop. It should work and hopefully give you an idea of where the issue must stem from.
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