SQL Server backups over the network fail with operating system error 32

Hello community,

I have Googled this topic to death and I cannot find the solution to my problem.  Here is my setup:

Database Server 1:
SQL Server 2014 Developer
WIndows Server 2012 R2 Std

Backup Server 1:
SQL Server 2014 Developer
Windows Server 2012 R2 Std

Backup Server 2:
SQL Server 2014 Developer
Windows Server 2016 Std Eval

On Database Server 1, I have 3 databases: DB1, DB2, DB3.

I have created the following script to back each one up (one-at-a-time) to one of the backup servers:

Mapping Drives:

exec xp_cmdshell 'net use P: /delete'
GO
exec xp_cmdshell 'net use Q: /delete'
GO
exec xp_cmdshell 'net use P: \\192.168.0.200\Backup_F'
GO
exec xp_cmdshell 'net use Q: \\192.168.0.200\Backup_G'
GO
exec xp_cmdshell 'dir P:'
GO
exec xp_cmdshell 'dir Q:'
GO

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Actual Backup:

DECLARE 
@today nvarchar(20),
@DB_to_backup nvarchar(128) = 'DB1',
@PATH_FILE1 nvarchar(128) = 'P:\SS_BKP1',
@PATH_FILE2 nvarchar(128) = 'Q:\SS_BKP2',
@backup_type nvarchar(128) = 'diff' /* Specify 'full' or 'diff' */

SELECT @today = SUBSTRING(REPLACE(REPLACE(REPLACE(CONVERT (datetime2,GETDATE(),120),'-',''),':',''),' ','-'),0,16)

DECLARE 
@DISK1 nvarchar(256) = @PATH_FILE1 + '\' + @DB_to_backup + '\' + @DB_to_backup + '_' + @today + '_' + @backup_type + '_MTS4Mb_BufferCount22_file1_of_2.bak',
@DISK2 nvarchar(256) = @PATH_FILE2 + '\' + @DB_to_backup + '\' + @DB_to_backup + '_' + @today + '_' + @backup_type + '_MTS4Mb_BufferCount22_file2_of_2.bak'

PRINT @DISK1
PRINT @DISK2

DECLARE 
@BACKUPSETNAME NVARCHAR(256) = N'' + @DB_to_backup + '-' + @backup_type + '-' + @today + '-MTS4Mb_BufferCount22'

IF @backup_type = 'diff'
	BACKUP DATABASE @DB_to_backup TO  
	DISK = @DISK1,
	DISK = @DISK2
	WITH DIFFERENTIAL, COMPRESSION, NOFORMAT, NOINIT,  NAME = @BACKUPSETNAME, 
	SKIP, NOREWIND, NOUNLOAD,  STATS = 1,
	MAXTRANSFERSIZE=4194304,
	BUFFERCOUNT=22;

if @backup_type = 'full'
	BACKUP DATABASE @DB_to_backup TO  
	DISK = @DISK1,
	DISK = @DISK2
	WITH COMPRESSION, NOFORMAT, NOINIT,  NAME = @BACKUPSETNAME, 
	SKIP, NOREWIND, NOUNLOAD,  STATS = 1,
	MAXTRANSFERSIZE=4194304,
	BUFFERCOUNT=22;

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When I run this code, whether from a query window or from within a Maintenance Plan, the backup for DB1 fails midway (at any given percent) yet the backups for DB2 and DB3 always succeed.

This is what I will see in the Event Viewer for DB1 when it fails:

BackupIoRequest::ReportIoError: write failure on backup device 'P:\SS_BKP1\DB1\DB1_20180629-124841_diff_MTS4Mb_BufferCount22_file1_of_2.bak'. Operating system error 32(The process cannot access the file because it is being used by another process.).

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System 

  - Provider 

   [ Name]  MSSQLSERVER 
 
  - EventID 18210 

   [ Qualifiers]  49152 
 
   Level 2 
 
   Task 6 
 
   Keywords 0x80000000000000 

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When I Google for solutions to this error, everyone suggested to look for whichever process is accessing the same file when the backup aborts.  While I have the required programs (procexp.exe and handle.exe from Sysinternals), the problem is that as soon as the backup fails, the partial files automatically get deleted, so i cannot check anything regarding them, let alone locks and handles from other processes.

However, I started the backup again and queried for handles (on Backup Server 1) while it was still running.  The results were hardly heartening or surprising.  I ran:

handle.exe -a > C:\current_handles.txt

And searched the resulting dump file for the folder where the said backups were supposed to save, and I found these open handles under these processes:

System pid: 4 \<unable to open process>
  B78: File  (---)   F:\Backup\SS_BKP1\DB1\DB1_20180629-145347_diff_MTS4Mb_BufferCount22_file1_of_2.bak
 1294: File  (---)   F:\Backup\SS_BKP1\DB1\DB1_20180629-145347_diff_MTS4Mb_BufferCount22_file1_of_2.bak
------------------------------------------------------------------------------
explorer.exe pid: 9120 NETWORK\Administrator
  604: File  (RWD)   F:\Backup\SS_BKP1
 14E8: File  (RWD)   F:\Backup\SS_BKP1

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I think it's safe to say I can't KILL these processes to free up those handles.

Other things I've tried to prevent any potential locks or conflicts was to disable Volume Shadow Copies on the Backup Servers as well as the Backup Server's local SQL Server instances.

The backups still fail.

I also modified my script to save on the local hard drive instead of the backup server and that did NOT fail, however, that is not practical on an ongoing basis.

I manage 15 database servers with 65 databases.  Only 2 of the servers are giving me this headache, and of those two, 1 is just partially failing.

Any and all help would be greatly appreciated!