Hi Martin,
The 'upload file' button is greyed out.
The upload button is greyed out when you have reached your storage limit.? You should get a notification when you are at 80% and 100% of your limit but that feature is not working at present.? See
/g/GroupManagersForum/message/15387 for more details.
Storage usage overview is shown in Admin > Billing for a main group or Admin > Usage for a subgroup.
Regards,
Andy