Hello,
I am using 1.5.6.2 with Transformer Theme 2.6.6 (a GREAT and robust them with awesome support!).
It's a Prestashop "bug", there's a temporary solution.
Edit the /themes/transformer/header.tpl, line 147
From <img class="logo" src="{$logo_url}"
To
Maybe other solutions exist... I don't know, but this one works.
Regards.