Hi, @Mariusz Siedlecki Welcome to the Microsoft Q&A forum, Thanks for posting your question.
The document xp_loginconfig says this is only supported for SQL Server and has the note to avoid using this feature,
Similarly to other extended stored procedures xp_loginconfig is on a deprecation path and is not supported on Managed Instance:
For more information please refer here
Hope this will help. Please let us know if any further queries.
Regards
Geetha
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