I experienced a similar problem.
I uninstalled the 7.0.100-preview.5.22307.18 sdk using add remove programs and then changed the TargetFramework in the project file to use 7.0 and then I was able to load the projects.
I experienced a similar problem.
I uninstalled the 7.0.100-preview.5.22307.18 sdk using add remove programs and then changed the TargetFramework in the project file to use 7.0 and then I was able to load the projects.