Hi @ralf.kronen,
You can click on the three dots next to the collection and click on ‘Modify permissions’
Then, you modify the permission for your other user to ‘can edit’ and it should work.
Update:
After the rollout of Postman 7 which included the support for RBAC (Role-Based Access Control), the view and functionality has changed.
Now, you’ll have to assign roles to a user regarding what all permissions you need to give them.
The following doc has all the details: https://learning.getpostman.com/docs/postman_pro/managing_postman_pro/roles_and_permissions
Let me know if you have any questions regarding this.