I had the same issue.
Database backup was on another server (server X). I tried restoring that backup on server A.
I was able to access backup folder (not the shared folder but actual folder using \\serverX\v:\mssql\backup) using my windows login (which were also used as windows authentication to login sql serverA)
Specifying the address gave me error that SQL Server was not able to access the location specified or there is issue with access rights.
Then I tried using the shared folder address ie \\ServerX\backup - still got the same message
Then I gave the account under which MSSQL service was running - READ rights on shared folder in ServerX. After this, when I gave shared folder address to restore database, it went on smoothly