Check the upload message and the account’s Vault usage under Settings → Account.
- Complete recovery-email verification; an unverified account has only 1 MB of storage.
- Make sure the file fits within the remaining plan quota.
- Rename files that contain characters rejected by the destination filesystem.
- Keep the browser tab open until the transfer completes.
- For a large file, use the Nextcloud desktop client so an interrupted transfer can resume.
If only one shared folder rejects uploads, the share might not grant create permission or the owner’s quota might be full.