load up a 5.4 project and be greeted by sprite instances that have lost their material - now that’s what i call alpha! 
Load up a 5.5 project and have the same error: Failed to unload ‘ProjectSettings/UnityAdsSettings.asset’
After update, load a 5.4.1f1 project under 5.5.0f3 :
2 warning
Platform MacStandaloneSupport does not support graphics device type OpenGL
Platform LinuxStandaloneSupport has only one supported graphics device type (OpenGL Core)
And the same error
Failed to unload ‘ProjectSettings/UnityAdsSettings.asset’
Same error on 5.5.0f3.
In the console log is says:
Refresh: elapses 0.330112 seconds
Refresh: trashing asset 00000000000000008100000000000000
Failed to unload ‘ProjectSettings/UnityAdsSettings.asset’
Then Unity Crashes when I run it.
!!!This is happening on any project I try to load from a saved version of any previous Unity Project.!!!
This is a major issue.
I finally got it to run by disabling the Unity Ads script.
Very strange.
You should report issues not related to experimental features in the base release, in this case 5.5.