Install Android SDK command line tools (zip file), specific version 25.2.3. From this version I was able to run the SDK manager (tools/android.bat) which allowed me to install Platform-tools 26.0 and Build tools 25.0.3
I downgraded the JDK to 1.8!
Set the paths right in Unity - preferences - external tools
See: https://forum.unity.com/threads/unity3d-5-5-0fx-unable-to-list-target-platforms-error.446096/#post-2977689
Unity 2017.1 AndroidSDK 8