Can't import users from Active directory

Can't import users from Active directory

I have Service Desk Plus Version: 5.5.0 Build Number: 5500 but I can't import users from Active Directory.
We have external and internal domain. Users from internal domain are imported successfully.
But when I connect to external domain controller, I can see users, and I also can choose them. Import Wizard connect with the AD but can't import the users (failed records). What can I do?

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