Web API Post returning error "Operation is not valid due to the current state of the object"

I have a .Net Core Web Site calling a ASP.Net web API to insert data and a document into an Oracle database. On clicking the submit button and invoking the post method, I get the error: Operation is not valid due to the current state of the object. Any idea what this is? I have found a couple of things, but no fix yet. The first thing I came across was adding a key to appsettings:

<appSettings>
      <add key="aspnet:MaxHttpCollectionKeys" value="2001" />
</appSettings>

But that had not effect.