Symptoms

The sync fails and produces a log line similar to the following:

Error during Sync occurred: Object reference not set to an instance of an object.,    at ILST.Group.FindGroups(List`1 allUsers, List`1 uniqueIdentifierNames, ConnectionContainer connections, ParserType IdType, String ADType)

Diagnosis 

The <connectionId> tags in a block of the config file are misspelled or missing, or the Id number is invalid.

Actions

Customer Actions:

Check that all <UserAccount> and <Group> sections have a valid connection ID as defined in the <Connections> block.

The default is "<ConnectionId>1</ConnectionId>"