Hi @Marcel Slats ,
Something here might help:
https://stackoverflow.com/questions/64325574/devops-organization-disappeared-after-linking-to-azure-active-directory
They're also actively answering in the dedicated Azure DevOps forum over here:
https://developercommunity.visualstudio.com/spaces/21/index.html
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(If the reply was helpful please don't forget to upvote or accept as answer, thank you)
Best regards,
Leon