Iothub module authentication error in Azure - azure-iot-sdk

Question

I am running a fairly basic Azure IoT C# module. Initially, it all worked fine. However, when I set up X.509 certificates for the edge hub, I've been getting module authentication errors as follows. There seems to be some dependence on using Tcp_only or Websocket_only options of the AMQP/MQTT protocols (where the latter does not throw the exception), but this is not consistent across modules.
The offending lines of code are below
..
// Open a connection to the Edge runtime
ioTHubModuleClient = await ModuleClient.CreateFromEnvironmentAsync(settings);
await ioTHubModuleClient.OpenAsync();
The error message is below. Essentially the await times out because authentication is not complete. Far as I know, authentication is derived directly from the iot edge runtime, so not sure why it was specifically trying to authenticate or erroring out...
Unhandled Exception: System.AggregateException: One or more errors
occurred. (The remote certificate is invalid according to the
validation procedure.) --->
System.Security.Authentication.AuthenticationException: The remote
certificate is invalid according to the validation procedure. at
Microsoft.Azure.Devices.Client.InternalClient.<>c.b__62_2(Task
t) at System.Threading.ExecutionContext.Run(ExecutionContext
executionContext, ContextCallback callback, Object state) at
System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task&
currentTaskSlot) --- End of stack trace from previous location where
exception was thrown --- at
System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task
task) at SampleModule.Program.d__10.MoveNext() in
/app/Program.cs:line 69 --- End of inner exception stack trace --- at
System.Threading.Tasks.Task.ThrowIfExceptional(Boolean
includeTaskCanceledExceptions) at
System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout,
CancellationToken cancellationToken) at
SampleModule.Program.Main(String[] args) in /app/Program.cs:line 41