If for any reason an OCS 2007 or R2 front-end server unexpectedly breaks down and stays out of service for a long duration, users that are home to the unavailable server will not be able to sign in to the system.
When you try to transfer affected users using the Move option on the right-mouse context menu or select the “Server or Pool” drop down box on the Communications tab of the user properties, an error will result:
Cannot perform the move user operation because the source pool cannot be contacted. Please check the event logs for more information.
To fix this, fire up ADSIEdit, locate the user, navigate to the msRTCSIP-PrimaryHomeServer attribute and Edit the FQDN to point to an available server or pool accordingly:
CN=LC Services,CN=Microsoft,CN=ocsr201,CN=Pools, CN=RTC Service, CN=Microsoft, CN=System,DC=swissitpro,DC=local
Obviously, manual modification by hand for multiple users is not very productive. Consider using a tool such as ADModify.NET or write you own customized PowerShell script. Take a look at the OCS 2007 Resource Kit or the Office Communication Server 2007 Powerpack.