Repeating message

Seem to get this message every time I open up. Not sure what it means and if I need to be concerned.
I’m a “free user” and just moved from userfriendly WLM.
Thanks Peter Scott

09:25:56 MailClient.Storage.Application.OperationException: Uploading item(s) to folder ‘’ failed due to the following error: Execution of request failed: (Invalid XML Document.)