MS Exchange: Proxy Error after switching to MS Graph Authentication

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Hi everybody,

sorry to bother you with this: One of our customers switched the authentication for his MS Exchange Online to MS Graph and since then receives the error shown in the screenshot attached.

He does not run any kind of proxy server on his firewall, so that we are confused by the error message.

Any hint is welcome to get his productive system up and running again.

Cheers, Ingo