.NET web application uses C# Driver 2.3.0 to access MongoDb. App runs on an EC2 instance in my AWS account. Instance is in a VPC that is peered to the Atlas VPC. Network ACL allows all traffic to/from the ATlas VPC. SEcurity group is wide open.

IPs of my VPC are whitelisted in the Atlas security config.

From my EC2 instance, I am able to connect to the Atlas instance from the mongo shell. A PING of the Atlas hostname resolves to the correct IP.

Applicatoin was working as expected earlier in the day. No code or configuration changes occurred. Application is now not able to connect to the instance. Following error occurs:

A timeout occured after 30000ms selecting a server using CompositeServerSelector{ Selectors = ReadPreferenceServerSelector{ ReadPreference =

{ Mode = Primary, TagSets = [] }

}, LatencyLimitingServerSelector

{ AllowedLatencyRange = 00:00:00.0150000 }

}. Client view of cluster state is { ClusterId : "1", ConnectionMode : "ReplicaSet", Type : "ReplicaSet", State : "Disconnected", Servers : [{ ServerId: "

{ ClusterId : 1, EndPoint : "Unspecified/sts-armor-01-shard-00-00-42ma8.mongodb.net:27017" }

", EndPoint: "Unspecified/sts-armor-01-shard-00-00-42ma8.mongodb.net:27017", State: "Disconnected", Type: "Unknown", HeartbeatException: "MongoDB.Driver.MongoConnectionException: An exception occurred while opening a connection to the server. ---> System.Security.Authentication.AuthenticationException: The remote certificate is invalid according to the validation procedure.