eM Client is compatible with Windows 11, but this is a restriction imposed by Microsoft on using oAuth with some account types. It is something they should change, but maybe they want to discourage non-MS apps. If you need to use Exchange, there is a registry setting you can change so it does not use oAuth and that should work.
It is the same setting I described here: Office 365 login, Unusual Browser error - #19 by Gary
Once you have added the registry value, setup the account again.