With sa user there is an error in error logs:

"Cannot open backup device "\\xxxxxxxx\xxxx.bak". Operating System Error 5 (failed to retrieve text for this error. Reason: 15105). (Microsoft.SQLServer.Smo)"

 But when I want to restore with my system administrator user, there is an error:

Backup,Unknown,BackupIoRequest::ReportIoError: read failure on backup device '\\xxxxxxxx\xxxx.bak'. Operating system error 13(The data is invalid.).

I think other DBA take this backup manually from PROD with his own sysadmin account and in TEST sa password different. So it gives error with permission denied. And when I want to restore with my own sysadmin account it gives second error. 
I dont understand, is there a problem with permission or file corrupt?