This issue most probably because account specified in sql instance configuration window has no enough privilege to access use database.
To back up sql server database, account should has at lease "sysadmin" role, you may change account to anther one that has "sysadmin" role or add "sysadmin" role to current account.
To add "sysadmin" to backup account , please do following :
1 . Open SQL Server Management Studio and navigate to "Security->Logins".
2 . Select an account that is specified in instance configuration window. If your server uses Windows authentication mode , please select "NT AUTHORITYSYSTEM" , because DataBK SQL Server Backup Agent runs as a serivce ,and the default logon account is this one.
3 . Mouse right click on selected account and select "Properties" , then select "Server Roles" , make sure "sysadmin" is selected .