This error is permissions related, and may be due to the fact that Active Directory can take about 15 minutes to propagate changes from one Domain Controller to another.
If your Exchange Server and main domain controller are different machines, it is possible that the permissions required to do the Mailbox backup will take some time to propagate to your Exchange Server.
Check permissions
Make sure that the permissions to perform the backup are correctly assigned to your backup user. Instructions can be found here: http://support.microsoft.com/kb/823018/en-us
Wait for propagation
The Microsoft literature�advises that propagation of permissions can take up to 15 minutes.
BA1760 - Exmerge Log: Error creating EntryID for �(GetMailboxInfoFromAB)