Hello,
I have several concrete 5.6 on a Windows 2012 server (IIS 8), PHP 5.6, no problem, works flawlessy.
I'm testing Concrete 5.7.5.6 (latest at time of writing), all is working except speed (other post) and extensions download/update, that I'm discussing here.
I can browse addons and extensions but at time of pressing "download" the page that states something like "error downloading package" appears (I'm on Italian language, don't know the exact english text) suggesting me that i should download and manually install the package.
Then I have a theme (fruitful) that was updated but trying to update nothing happens. I'm guessing it's related to the same issue.
What can I check for understanding what is failing? Is there some log, at level of PHP or what else?

Thank you to all who wants to help me.
Best regards,
Henrik Sozzi