Hello @ADM Jukka Lamminpää!
Thank you for raising the question.
DefaultResourceGroup-<region> are default resource groups that appear in each subscription.
The most encountered scenario I personally met, is when you create the AKS cluster with monitoring addon enabled, it will need a Log Analytics Workspace that is part of DefaultResourceGroup-<region>. The same would apply if you would enable Microsoft Defender for Containers.
If you are creating the AKS cluster with monitoring addon enabled or with Microsoft Defender for Containers enabled, I would recommend you to re-create 'DefaultResourceGroup-NEU' or specify another Log Analytics Workspace during the AKS cluster creation.
If your external developer is not creating the AKS cluster with monitoring addon enabled or with Microsoft Defender for Containers enabled, please let me know the parameters used to create the AKS cluster (make sure NOT to share any confidential information).
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Thank you and happy new year!