WordPress on Windows - plugin updates do not download

I have a WordPress installation on a Windows Server that we manage. One issue that we've come across is that if you click the Update button on a plugin, the in-progress icon spins, but the update never completes.

I found that the updates were actually downloading to Windows/Temp dir but never got beyond zero bytes. In wp-config, I changed the destination of the downloads to a local dir:

define( 'WP_TEMP_DIR', ABSPATH . 'wp-content/tmp/') ;

...which worked -- except that the downloads are still not getting past creating the zero byte file. So it's not a permission issue as far as I can see, since the file is created (advanced-custom-fields.4.4.3.tmp for example).

It just never actually downloads.

Any ideas? Is this a CURL issue? FTP? I'm not sure what mechanism WordPress uses to download the update files behind the scenes.

Thanks!

via Chebli Mohamed