According the Microsoft, if the error is "Access Denied", then it is because the BUILTIN\Users group on the DC is missing the NT AUTHORITY\Authenticated Users group. Go to the DC and add the NT AUTHORITY\Authenticated Users group to the BUILTIN\Users group then restart the srmsvc and srmreports services.
As of WS08 R2, FSRM attempts to authenticate against an Exchange Server using the computer account (domain\computername$ format) account of the server. This computer account must be granted send as permissions on the mailbox that you are trying to send as, or it will fail with this error (where the same configuration will work on WS03 R2 without failing, assuming your receive connectors are already configured correctly).
I ran the following Powershell command on my Exchange (2007 SP3) server to grant the necessary permissions on the mailbox I was trying to send as, after which it began working:
Add-ADPermission -Identity "Mailbox Display Name" -user "Domain\ServerName$" -extendedrights "Send-as"
You can also substitute by using a domain group that contains the server computer accounts you want to allow.
for information on solving this problem.
From a newsgroup post: "One of my staff members called Microsoft and they said that this is a bug that they are working on. You can turn the error off by going to the "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SrmSvc\Settings" registry key and setting "DisableSearchDfs=1"".