I can't restore or forward messages in Mail Archiver

This functionality isn't turned on when you install Mail Archiver. In order to allows this, you will need to:

Open the console.
Select the Mail Archiver node.
In the “Exchange Mailbox Access” section click “Configure” and complete the wizard.
When the wizard has completed, check the “Enable Restore To Mailbox” checkbox.
Click Save.

If you are still having issues, ensure that you have checked the following:

EWS is used to restore messages, so make sure that this is functional.
Check the password hasn't expired for the Mail Archiver account. This is shown on the Mail Archiver node. If it has, reset the password in Active Directory Users & Computers, follow the First Setup Wizard again and input the new password.
Make sure that you also check the Exclaimer event log, which can be found under Application and Services Logs.