WordPress: Download failed. A valid URL was not provided.

Some days ago I’ve updated my WordPress to the new 3.8 version.
For storing my themes and plugins I’ve set up a repository on a virtual machine running on the same virtual network.

This was working perfectly until WordPress 3.5, but since 3.6 the developers added a new security feature that denies access to repository with a local ip, for example 172.[16-31].x.x as in my case, but also 127.0.0.1, 10.x.x.x, 192.168.x.x.
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