Backup Exec 2012 Backup Exec Device & Media Service stop working and won't start

Backup Exec reports No available Media or device.  When I try to restart services, Device & Media Service won't start, logon failure, I have done this: Created a new account in Windows and added it in Backup Exec, works just fine for a day or two, and then the same problem. Gone back to Administrator account, after a day or two it happens again. I can restart the Device & Media service in Windows, just retype the password, and it works fine again. Backup Exec is updated and installed on an updated Windows 2008R2, Backup Exec is trial for the moment.