This issue most probably because account specified in sql instance configuration window has no enough privilege to access use database.
Solution :
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 backup a sql server database to a network shared folder , all you need is to add a backup device to target shared folder.
Storage server saves backup to target shared folder , but Storage Server service is running as Local System account in default . Local System account may not have enough privileges to access the network shared folder. So you can change DataBK Storage Server service account to a new one in in Control Panel->Administrative Tools->Service , see following : (Please note that : Server that shares the folder should also has the same account name and password . ) 
