Reproduction steps:

1. Set-up file sharing (Either with a Static IP address or with Machine Name)
2. Share a file (Image)
3. Open "MinimalRepro.zip" project
4. Open "Main" scene
5. Open "WebRequestTest.cs" script
6. Edit line 18 (Or line 39) to use the URL you shared the file with
7. Enter Play mode

Fixed in: 2018.2.0a1

Expected Result: The image loads successfully
Actual Result: "Generic/unknown HTTP error" is outputted in the console and the image isn't loaded

Reproduced with: 2018.1.0b3, 2017.3.0p2, 2017.2.1p2, 2017.2.0b1
Did not reproduce on: 2017.2.0a4, 2017.2.0a1, 2017.1.3f1, 5.6.5f1