Maybe a different process has access on this file/folder at the same time and is therefore blocking the access through QlikView. You could try to move your start-times a bit maybe 1 - 5 minutes before or after your now scheduled start-times.
If it doesn't help you will probably need to look into the document-log of the qvw, the windows event- and security-logs and the logs from your security-application to find a hint on the process which locked the qvd.