Local Storage is full after failed ISO upload - gorect/Proxmox GitHub Wiki

In the event that an ISO upload has failed due to low storage space, Proxmox will not remove the failed data, clogging up your storage. If you believe that this is the case you just need to navigate to /var/tmp and look for any temp files that were left behind. These are the files that will not show up in the ISO window in the Proxmox admin page.