If you get this issue, you need to check that the email account is open on the server, and is running in the same session as the Task Server.
Once you've done this, if you get the error below, please complete the following steps.
Log into user profiles in Proclaim then click the Authorisations tab.
Select the following check boxes:
Don't Show Mailbox in Proclaim Desktop
Don't Show Calendar in Proclaim Desktop
Log out of the task server user account then log in again. The message should no longer appear.
Restart Task Server and test.
If the above fails try again with the following System Parameter Maintenance:
Key: Outlook
Code: no direct access
Value: YES