Hi Andreas ,
Actually, there is no need to check it. I'd like to confirm some additional information to move forward.
Let' focus on one internal user (User A) first.
1.Please click the share with everyone folder -> copy the URL and send it to User A -> Let him access the files in this folder directly via the URL to check the outcome.
2.If User A can't access, please click check permissions on the screenshot you attached -> type User A -> Check. Please capture a full screenshot of the permission.
3.Please let me know how these internal users access the shared with everyone folder.
We will keep assisting you based on your updates.
Regards,
Sky