Keep this tab open to retry. If an upload's outcome is unknown, check your files before retrying to avoid a duplicate.
Make something for your reMarkable.
This app talks to the reMarkable cloud on your behalf. To do that it needs a one-time code, which it exchanges for a device key that this browser keeps; the server holds no copy. You do this once per browser.
The key grants full access to your reMarkable cloud files. It never leaves this machine — revoke it any time from your device list.