We have a problem with our Exchange 2016 setup. It´s setup using the iApp and nothing has been changed.
But we have a recurring probelm where some users get a loginprompt using outlook application.... just wondering if anyone in here had any similiar types of problems?
It sounds like the NTLM Authentication is failing so you should troubleshoot that. I would start, but setting the APM logs on informational and see what you can find for when these users (Sessions) fail.