A different user name is the common cause for this, Nuget downloads everything into: "C:\Users\USER_NAME\source\repos"
and if you had the project previously setup on a different user name the .csproj file may still contain that old user name there, simply open it and do a search replace for "C:\Users\_OLD_USER_NAME\source\repos"
to "C:\Users\NEW_USER_NAME\source\repos"
.