You may want to to check the headers you are sending, based on the documentation the groupuserpicker is expecting Accept: application/json, not Content-Type.
Great point, thank you Mikael ! But still "403 You are not authenticated. Authentication required to perform this operation." Does anything else occur to you ?
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No nothing else looks odd, I just know that some endpoints are particular when it comes to the headers, for example there are Confluence endpoints that expect Content-Type instead of content-type even though headers should not be case sensitive.
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Update the permission Issue seems to only be related to any request related to users. I am without any issue able to fetch projects or a single project using: /rest/api/3/project
Could it be the case that I should probably ask somewhere for some additional permissions?
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