I had exactly the same issue following the 16.9.1 update. After much frustration and trying everything I could think of including the suggestions in the other answers I found that this is actually a known issue introduced by Xamarin.Android 11.2.0.21 which ships with VS 16.9.1.

The problem seems to be caused by an update to fast deployment which requires .mdb debug files instead of the old .pdb symbols for Android projects.

To fix you need to update any NuGet packages which still use the old pdb symbol files in the hopes that they have been changed to use mdb debug files. In my case this wasn't possible, so the workaround is to add the following lines to your Android csproj just above the </Project> tag:

<Target Name="_RemoveMdbFiles" BeforeTargets="_Upload">
<ItemGroup>
    <_Temp Include="%(_ResolvedSymbols.Identity)" Condition="'%(_ResolvedSymbols.Extension)' == '.mdb'" />
    <_ResolvedSymbols Remove="@(_Temp)"/>
</ItemGroup>

If this workaround also isn't an option for you, I think the only other resolution would be to downgrade to VS 16.8.x.

EDIT: Seems to be fixed in VS 16.9.2 update.

Src: https://docs.microsoft.com/en-us/xamarin/android/release-notes/11/11.2#known-issue-fast-deployment-and-mdb-files
https://github.com/xamarin/xamarin-android/issues/5631#issuecomment-794289675

A workaround is to create a file called Xamarin.Auth.dll.mdb. I just created a copy of Xamarin.Auth.dll and renamed it. It does not solve the underlying problem but at least I'm able to debug. Not sure if this have any side effects but so far so good...