apparently it's not possible to create a new plan for an existing group, if the creation of groups is disabled in Azure AD.
I disabled the creation of groups for normal users (https://docs.microsoft.com/en-us/office365/admin/create-groups/manage-creation-of-groups?view=o365-w...). Only administrative groups are allowed to create Office 365 groups in the tenant now.
Teams (and so Office 365) are created by the IT department. End users can't add new Plans to their Teams, even if they are the Owner of the team.
In my opinion this is unexpected behavior. Is this really intended?
EDIT: Apparently it's possible to add new plans to existing groups, that already have atleast one plan added. I guess there are some processes in the background, that add attributes/infos to the Office 365 Group mailbox and Sharepoint team site when adding the first plan to a group.