Causes may be that any other process blocked the access or that there aren't proper access rights or that the file isn't really there respectively available (means the file-entry within the storage-system might be set but the file-content itself may not completely available - such tasks might queued and/or splitted ...).
Just to exclude any specific issue to this qvd I suggest to rename them with something like 'XXX...XXX.qvd' and copy & paste the 2020.qvd and rename it as 2021.qvd and loading again.
