Load failed for all projects in sln VS 2015

[nathvi]

I'm using:
Microsoft Visual Studio Community 2015
Version 14.0.25431.01 Update 3
Microsoft .NET Framework
Version 4.7.03056
Installed Version: Community

After copying the source code from Github and opening the sln file, every single project file says "load failed". I tried right clicking and reloading one of the projects but I get this cryptic message "The default XML namespace of the project must be the MSBuild XML namespace. If the project is authored in the MSBuild 2003 format, please add...". I'm not going to write out the full message because it's long and obnoxious.

I did notice that this project will build with the 2017 community edition of VS.

Any ideas here?