Reason:
SQL Server service must have access permissions on the folder where the SQL database backup file is. The error “Cannot open backup device – Operating system error 5” is probably because the SQL Service cannot read the backup file there.
To check on which account SQL Service/SQL Server Agent is running
Get the service account name for SQL Server service. You can use “SQL Server Configuration Manager” to gather Service account details
1) Launch SQL Server Configuration Manager – you should see the short cut in Start -> All Programs -> Microsoft SQL Server 2008 -> Configuration Tools -> SQL Server Configuration Manager
2) Expand tree view “SQL Server Services”. On right pane, youl will see the list of Services installed. Look at the SQL Server service specific to that instance. Chekc the column “Log on As” – This will give you the SQL Server service account context
3) Grant permissions for the above account to drive / folder that you are backing up files to . Please refer following articles on how to grant file / folder permissions in windows
Datapedia @JM_Arun 
The registry errors and incomplete installation of software also causes the problem. After a long googling I found this article here the term SQL Server Backup Error 15105 described in borad. http://sqltechtips.blogspot.in/2015/12/backup-error-15105.html