The Umbraco back office credentials are admin@admin.admin / adminadmin.
Try rebuilding the solution a few times.
If that doesn't work, see this: https://stackoverflow.com/a/34391473/2052963
It says to run this command in the Package Manager Console (in this case, with the "Website" project selected as the default project):
Update-Package Microsoft.CodeDom.Providers.DotNetCompilerPlatform -r