Workaround for bugged .tfignore
A bug in NuGet may be causing the problem. This is the workaround:
Create a .nuget folder in your solution folder, if it doesn't exist.
In the .nuget folder, create a file nuget.config with this content:
<configuration>
<solution>
<add key="disableSourceControlIntegration" value="true" />
</solution>
</configuration>