Thank you for your reply. I double-checked, and I’m sure it’s the user. I don’t see any problem with his account, except that this account uses an old type of userAccountId as described here - Rest API | custom field format for single-user and multiple-user is out-of-date - #7 by ibuchanan.
What’s strange is that this user sent the request from a browser to this endpoint and received data from the endpoint without any issues."