I REGISTERED BUT I CANNOT UPLOAD FILES.
Due to the high utilization of our infrastructure, new users are registered without quotas. In order to get more quotas and upload files to Pithos, they will need to participate in a project.
You can find more information in the relevant blog post.
However, if you are member in a project and you have available quotas you probably need to reassign your pithos container. You can find details on how to reassign any container here.