This weird bug was sleeping bomb in my clean install of PS 1.7.7.1 until I tried to configure Product Comments module (v4.2.1) that was installed along with PS itself.
I've got blank screen and 500 server error in Developer tools. After manually turning on dev mode and error reporting I've got following message:
a bit of googling gave me the idea that problem somehow related to Symfony's understanding of /src/PrestaShopBundle/Resources/config/services/core/file.yml
So, here is problem part of that file highlighted:
fixed version that worked for me:
Not sure, if it will lead any errors in future, but for now it works. Would like to hear back from developers if current solution is fine, or I should fix it some other way?