Exchange Web Service: False message of ‘Unable to read data from the transport connection’

Exchange Web Service: False message of ‘Unable to read data from the transport connection’

So we are using Exchange on-premise Web Services (/EWS/Exchange.asmx) to send Calendar Invites (S+) automatically when people register for trainings, but on the recent scheduled job, suddenly the C# App throws errror: [crayon-5ed2de443477e778298171/] First thing that we want to check of course is Firewall. Nope, I can browse https://exchange/EWS/Exchange.asmx just fine. Second thing is to blame Exchange, maybe there's a recent change? Since the app does not change at all. But going through IIS Logs of Exchange CAS is troublesome (the log files are 200MB daily!) Anyways, we could not see the IP trying to connect to Exchange Web Service....