botframework-sdk: Direct Line API + Action Fulfillment in LUIS not working

I was normally using this setup (Direct Line API + Action Fulfillment in LUIS) until yesterday when it started to fail. I have a LUIS model configured with action fulfillment using the Definition Channel. Then I published this model using the option “Enable Action Binding using Microsoft Bot Framework”, and registered a new bot in the bot framework portal to point to that model. Then yesterday, I started to receive errors when I post a message to the bot using the directline API. Next are the responses I get using different ways:

• Using the REST interface provided in the documentation here, I’m able to create conversatons and retrieve messages but when I post a new message I get a Response Body

{
  "message": "Failed to send message: bot returned an error"
}

with a Response Code 500

• When using the Direct Line Nuget package from CSharp code, I get

Microsoft.Rest.HttpOperationException occurred
  HResult=-2146233088
  Message=Operation returned an invalid status code 'InternalServerError'
  Source=Microsoft.Bot.Connector.DirectLine
  StackTrace:
       at Microsoft.Bot.Connector.DirectLine.Conversations.<PostMessageWithHttpMessagesAsync>d__7.MoveNext()
    --- End of stack trace from previous location where exception was thrown ---
       at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
       at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
       at Microsoft.Bot.Connector.DirectLine.ConversationsExtensions.<PostMessageAsync>d__5.MoveNext()
    --- End of stack trace from previous location where exception was thrown ---
       at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
       at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
       at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
       at Test.Directline.<GetDefinition>d__6.MoveNext() in C:\Test\Directline.cs:line 136
  InnerException:

Looking into Fiddler, I get the same error as in the REST interface.

• If I configure the bot to get telemetry using an Application Insights key, I get

Exception type: System.Exception
ExceptionDetail: 
{ "Data": {},
 "Source": "Microsoft.Bot.ChannelConnector",
 "HResult": -2146233088
 }
StatusCode: BadRequest

Any idea of what might be happening? Thanks!