Steps to reproduce:
1. Open attached "errorWWW.zip" Unity project
2. Open scene “Test”
3. Click on “GameObject” object in the Hierarchy tab
4. Manually enter “skyNight001.png” path on your computer into “File Path” field in the Inspector tab
5. Enter Play mode

Reproduced in: 2017.3.0a1, 2017.2.0b4, 2017.1.0p1, 2017.1.0f3, 2017.1.0f1, 2017.1.0b3, 2017.1.0b1

Didn’t reproduce in: 2017.1.0a6, 2017.1.0a5, 2017.1.0a1, 5.6.2p4, 5.6.1f1, 5.5.0f1

Regression introduced in: 2017.1.0b1

Error:
UriFormatException: Invalid URI: The hostname could not be parsed. ( )
System.Uri.Parse (UriKind kind, System.String uriString)
System.Uri.ParseUri (UriKind kind)
System.Uri..ctor (System.String uriString, Boolean dontEscape)
System.Uri..ctor (System.String uriString)
UnityEngineInternal.WebRequestUtils.MakeInitialUrl (System.String targetUrl, System.String localUrl) (at /Users/builduser/buildslave/unity/build/Runtime/WebRequest/Managed/WebRequestUtils.cs:481)

-------------------------
This bug is by design:
The newer Unity versions are more strict about URI conformance.