Hi @Mark Gray ,
To narrow this issue, I have something to confirm with you.
- Go to Microsoft 365 admin center Active groups page, make sure "Accounts and Added users" group is active and group type is Security. Generally we will get this error only if when we share a folder to a Distribution group.
- Click "Advanced" button and grant permission to the group(stop inheriting permissions before this), see if it lets you proceed.
- Recreate a folder and manage access to "Accounts and Added users" group to test if it only happens to a specific folder.
- Recreate a new security group and grant it access to "Accounts" folder to verify if it only happens to a specific security group.
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