Here is another strange but interesting issue, I faced almost a month back. One of my acquaintances was trying to import a CRM Organization [They exported the database and were importing the database into an existing CRM installation] which was using a different domain. Everything was going fine until he reached to the point where you are supposed to map existing users with users from destination AD. The moment he reached that stage he was running into an Exception in Deployment Manager add-in [See the screenshot below].
After trying to troubleshoot this problem for a while, I found few pointers regarding empty domainname field in SystemUserbase table. Upon examination of the SystemUserbase table, we found that indeed there were few records with empty domainname field in there. Now INTEGRATION and SYSTEM are valid users of CRM and they do not have domainname mapped to them however, for every other record there must be an associated user from AD i.e. domainname field must be populated. In our case, there were few other users, which had empty domainname field. We filled in those fields with valid domain\username information and voila, import went just fine.