HttpClient throwing "An error occurred while sending the request."

I face the exact same problem in production environment. Similar setup and about 30k http calls send from the client. Errors occur in a very infrequent way and it is very difficult to reproduce.

After reading numerous posts I think this Microsoft's bug when doing the connection pooling in HttpClient (in my solution I use httpClientFactory). You can take a look here

The approach followed to overcome this issue until Microsoft fixed the issue:

1) Retry policy. Polly has been used and when this exception occurs the call is retried after a couple of seconds.

2) Increased the request timeout to 3 minutes.

It seems to work fine for now but as I mentioned before it is hard to reproduce the error in a controlled way.