Had this error a few times and finally got a solution
Operating system error 5(failed to retrieve text for this error. Reason: 15105).
BACKUP DATABASE is terminating abnormally
First be sure that your sp users got the required rights.
So share your backup folder on the SQL server and give your sp_admin, sp_farm and sqluser full rights
Open SQL Server Configuration Manager
Expand SQL Server Services
Go to the properties of your SQL Server instance and change the Log On As to Local System.
Restart SQL and try again