You cannot connect to the Exchange webmail (owa) which is hosted behind a reverse proxy (ARR, WAP, WAF …).
The other services (Outlook / Telephone) work perfectly.
You are in a high availability architecture (DAG) and a Reverse proxy – Load balancer is used.
The article discusses the use of IIS and ARR.
You must activate the “Client affinity” option to prevent the identification request from being “balanced” from one server to another.
Configuration on ARR: