When using OWA (internally or externally in this situation), when one either chooses to search the Global Address List or pages forward from the first 50 members of the list, OWA searches for a few moments and then returns the error: “Could not connect to a directory server. If the problem continues, contact technical support for your organization.”
This is a known issue and can be resolved using the following method.
To resolve this issue, either use Outlook lite (I did not actually try this) or, in the TMG Firewall Outlook Web Access publishing rule, go to the Link Translation tab and uncheck the “Apply link translation to this rule” check box and apply the changes. Wait for a few minutes and the GAL should be fully searchable.
Special thanks to Michael Hensley for the screenshot and Ken for the Solution.