Exchange access was working for many months. Suddenly this morning it is broken accessing my Office365 account. I am able to access using Outlook itself without changing a thing, but emClient simply stopped working.
As a Pro License user, it might be better for you to open a support ticket with eM Client.
Otherwise, if you want to continue here, go to Menu > Tools > Operations, and look for an error in the Log tab. Also, please give the eM Client version you are using.