Ok could be then a eg: corrupted OAuth token.
Suggest next to then “close eM Client” snd go online to your Outlook account third party app section and remove / revoke the eM Client token and then reopen eM Client and you should get a browser prompt to reallow access to your Outlook account with a new token.
Manage third-party app access to your Microsoft account by visiting the below site.
Privacy dashboard (https://account.microsoft.com/privacy) and navigating to “Apps and services” to revoke permissions for, or remove, unwanted apps.