I am trying to create a backup plan using the TFS Power Tools but I keep running into this error message:

I have checked that the account has Full Control on the share, I can edit, create and delete files there. From the log:

[Info   @07:15:00.403] Starting creating backup test validation
[Error  @07:15:00.700] Microsoft.SqlServer.Management.Smo.FailedOperationException: Backup failed for Server 'WMSI003714N\SqlExpress'.  ---> Microsoft.SqlServer.Management.Common.ExecutionFailureException: An exception occurred while executing a Transact-SQL statement or batch. ---> System.Data.SqlClient.SqlException: Cannot open backup device '\\wmsi003714n\sql dump\Tfs_Configuration_20100910091500.bak'. Operating system error 5(failed to retrieve text for this error. Reason: 1815).
BACKUP DATABASE is terminating abnormally.
   at Microsoft.SqlServer.Management.Common.ConnectionManager.ExecuteTSql(ExecuteTSqlAction action, Object execObject, DataSet fillDataSet, Boolean catchException)
   at Microsoft.SqlServer.Management.Common.ServerConnection.ExecuteNonQuery(String sqlCommand, ExecutionTypes executionType)
   --- End of inner exception stack trace ---
   at Microsoft.SqlServer.Management.Common.ServerConnection.ExecuteNonQuery(String sqlCommand, ExecutionTypes executionType)
   at Microsoft.SqlServer.Management.Common.ServerConnection.ExecuteNonQuery(StringCollection sqlCommands, ExecutionTypes executionType)
   at Microsoft.SqlServer.Management.Smo.ExecutionManager.ExecuteNonQuery(StringCollection queries)
   at Microsoft.SqlServer.Management.Smo.BackupRestoreBase.ExecuteSql(Server server, StringCollection queries)
   at Microsoft.SqlServer.Management.Smo.Backup.SqlBackup(Server srv)
   --- End of inner exception stack trace ---
   at Microsoft.SqlServer.Management.Smo.Backup.SqlBackup(Server srv)
   at Microsoft.TeamFoundation.PowerTools.Admin.Helpers.BackupFactory.TestBackupCreation(String path)
[Error  @07:15:00.731] !Verify Error!: Account GROUPINFRA\SA-NO-TeamService failed to create backups using path \\wmsi003714n\sql dump
[Info   @07:15:00.731] "Verify: Grant Backup Plan Permissions\Root\VerifyDummyBackupCreation(VerifyTestBackupCreatedSuccessfully): Exiting Verification with state Completed and result Error"

Any ideas?