Hi @Tim Friedmann ,
According to my test, even if I am the site administrator of the site, not the owner or member of the Microsoft 365 group, I will encounter the same issue.
Please go to Microsoft 365 admin center> Groups> Activate groups> select the associated group of the team site> check if you are the owner or member of the group, if not, add yourself to this group:
You can also click the "Members" option in the upper right corner of the team site, make sure you are in this group:
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