Could not load type 'Microsoft.Build.Framework.SdkReference' on project open in VS 2017 U1 (15.3)

1.- Open Developer Command Prompt for VS2017 as Admin

2.- Uninstall the Microsoft.Build.Framework from GAC (Global Assembly Cache)

gacutil /u Microsoft.Build.Framework

3.- Re-install the 15.1.0 version (correct path according to current VS2017 edition)

gacutil /i "C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\15.0\Bin\Microsoft.Build.Framework.dll"

Now the projects in solutions should load properly.

Use an adminstrator "Developer Command Prompt for VS 2017" to run

gacutil /u "Microsoft.Build.Framework, Version=15.1"

Please do not reinstall another version to the GAC. MSBuild should not be in the GAC, and installing it there will cause problems the next time Visual Studio updates MSBuild, because the file from the GAC gets preferred over the updated version delivered with Visual Studio.