Your server no longer allows username/password logins.
You will need to go to Menu > Accounts and remove the account then add it back again by clicking on Add Account and entering the email address in the automatic setup.
This will setup the account using oAuth, which does not use a username/password in eM Client.
I have a similar problem. Exchange account was set up as Gary suggests. Today the password on the exchange account was changed (Microsoft forced it). Now I get password required when emClient tries to refresh.
How do I update emClient. Or do I have to delete the account from emClient and reinstall?