Android Resource issues after upgrading to Xamarin Forms 2.0

I have recently upgraded 2 machines from Xamarin Forms 1.x to 2.0. One machine is OK and Xamarin Forms projects build fine for Android. The other machine is having dreadful issues.

We have tried uninstalling and re-installing everything - Xamarin, Android SDK, all NuGet packages - the lot - but get the same error messages.
It seems to be related to Android app compat stuff.

Steps to replicate are:

1. Create new Forms 1.3 project - compile and run - all OK
2. Upgrade project from Forms 1.3 to 2.0 - issues

Lots of errors like these:

2>d:\Documents\Visual Studio 2015\Projects\App4\App4\App4.Droid\Resources\Resource.Designer.cs(1290,119,1290,143): error CS0117: 'Resource.Styleable' does not contain a definition for 'SnackbarLayout_elevation'
2>d:\Documents\Visual Studio 2015\Projects\App4\App4\App4.Droid\Resources\Resource.Designer.cs(1291,130,1291,165): error CS0117: 'Resource.Styleable' does not contain a definition for 'SnackbarLayout_maxActionInlineWidth'
2>d:\Documents\Visual Studio 2015\Projects\App4\App4\App4.Droid\Resources\Resource.Designer.cs(1308,104,1308,113): error CS0117: 'Resource.Styleable' does not contain a definition for 'TabLayout'
2>d:\Documents\Visual Studio 2015\Projects\App4\App4\App4.Droid\Resources\Resource.Designer.cs(1309,118,1309,141): error CS0117: 'Resource.Styleable' does not contain a definition for 'TabLayout_tabBackground'
2>d:\Documents\Visual Studio 2015\Projects\App4\App4\App4.Droid\Resources\Resource.Designer.cs(1310,120,1310,145): error CS0117: 'Resource.Styleable' does not contain a definition for 'TabLayout_tabContentStart'
2>d:\Documents\Visual Studio 2015\Projects\App4\App4\App4.Droid\Resources\Resource.Designer.cs(1311,115,1311,135): error CS0117: 'Resource.Styleable' does not contain a definition for 'TabLayout_tabGravity'
2>d:\Documents\Visual Studio 2015\Projects\App4\App4\App4.Droid\Resources\Resource.Designer.cs(1312,122,1312,149): error CS0117: 'Resource.Styleable' does not contain a definition for 'TabLayout_tabIndicatorColor'
2>d:\Documents\Visual Studio 2015\Projects\App4\App4\App4.Droid\Resources\Resource.Designer.cs(1313,123,1313,151): error CS0117: 'Resource.Styleable' does not contain a definition for 'TabLayout_tabIndicatorHeight'

etc. etc.

Any ideas?