You have a User profile Service application configured in SharePoint 2010 /2013 environment with Active Directory Sync connection configured for Importing profiles. Once Imported the User profile as seen from "Manage User Profiles" link in User Profile Service application is seen as Domain\Username format by default.
There could be a situation when this show up in Domain: Username format instead of Domain\Username.
Note: For LDAP / FBA Claims scenario the profile or profile's Login name as shows MembershipProvider:Loginname. This is by default.
This occurs if the ObjectSid (AD attribute) to SID (SharePoint Attribute) mapping is missing from MOSS management agent. Typically, this may occur if the mapping has been removed from SharePoint >>CA>>UPA>> Manage User Properties link or via SharePoint Management Shell.
This Mapping is created while we Provision the AD Sync Connection by the Rules Extension DLL, as seen in "Configure Attribute Flow" section of AD management Agent & is stored in the Profile Sync DB. Once removed, it is permanently deleted.
While this is removed from UI, There is no warning message shown to End user indicating that this will cause issues & should not be removed.
This Mapping CANNOT be added back to existing SharePoint attribute SID by either UI (you do not get that option) or via SharePoint Management Shell.
The way out in this Situation is to reset the Sync DB (http://technet.microsoft.com/en-us/library/ff681014.aspx#resetSync) & then you would need to re-create the Sync Connections again. A Sync DB reset would warrant "My Site Cleanup Job” to be disabled and 2 Full + 1 Increment Sync to done.
Therefore, it is recommended to note the following details before proceeding to reset the Synchronization database.
1. Sync Connection Details as
a. Selected OU Details
b. AccountName used to Query AD sources
2. Any User /Group connection filters specified on Sync Connection
3. Custom Property Mappings for Additional Attributes
4. Configuration Settings for any BCS connector etc, along with Attribute Mappings