Error while trying to backup - Microsoft.SqlServer.SmoExtended, Error 5 (access denied)

Hey guy,

I have a problem with my SQL Express Server.

If I try to backup a database I'm getting the following error:

(Microsoft.SqlServer.SmoExtended)

System.Data.SqlClient.SqlError: Das Sicherungsmedium 'D:\my-database.bak' kann nicht geöffnet werden. Betriebssystemfehler 5(Access denied). (Microsoft.SqlServer.Smo)

I have done this a month ago and it worked.

Can someone help me please?

I am logged in as sa, the OS is Windows Server 2012 Essentials.

Thanks

Regards,

Alex