There's a new technical bulletin posted as well as a tool to help fix issues with the Active Directory legacyExchangeDN attribute.
See technical bulletin A2E_216 at:
In some cases, Exchange migrations may leave invalid Active Directory attributes on some mailboxes, preventing Add2Exchange from logging onto the affected mailboxes. The error above can be seen when when you are choosing accounts during relationship creation, in the Toolbox or during Disaster Recovery.
Fixing this issue entails correcting the legacyExchangeDN attribute of the affected account(s) within Active Directory. We have created a utility to assist in correcting the invalid legacyExchangeDN. A manual solution is also discussed.