Drive Redirection virtual channel hangs when copying a file from server to client over RDP 8.1

Question

Problem Summary:
A UTF-8 without BOM Web RoE XML file output from a line of business application will not drag and drop copy nor copy/paste from a Server 2012 R2 RD Session Host running RD Gateway to a Windows 7 Remote Desktop client over an RDP 8.1 connection and the Drive
Redirection virtual channel hangs.  The same issue affects a test client/server with only Remote Desktop enabled on the server.
Other files copy with no issue.  See below for more info.
Environment:
Server 2012 R2 Standard v6.3.9600 Build 9600
the production server runs RDS Session Host and RD Gateway roles (on the same server).  BUT,
the issue can be reproduced on a test machine running this OS with simply Remote Desktop enabled for Remote Administration
Windows 7 Pro w SP1v6.1.7601 SP1 Build 7601 running updates to support RDP 8.1
More Information:
-the file is a UTF-8 w/o BOM (Byte Order Marker) file containing XML data and has a .BLK extension.  It is a Web Record of Employment (RoE) data file exported from the Maestro accounting application.
-the XML file that does not copy does successfully validate against CRA's validation XML Schema for Web RoE files
-Video redirection is NOT AFFECTED and continues to work
-the Drive Redirection virtual channel can be re-established by disconnecting/reconnecting
-when the copy fails, a file is created on the client and is a similar size to the original.  However, the contents are incomplete.  The file appears blank but CTRL-A shows whitespace
-we can copy the contents into a file created with Notepad and then that file, which used to copy, will then NOT copy
-the issue affects another Server 2012 R2 test installation, not just the production server
-it also affects other client Win7 Pro systems against affected server
-the issue is uni-directional i.e. copy fails server to client but succeeds client to server
-I don't notice any event log entries at the time I attempt to copy the file.
What DOES WORK
-downgrading to RDP 7.1 on the client WORKS
-modifying the file > 2 characters -- either changing existing characters or adding characters (CRLFs) WORKS
-compressing the file WORKS e.g. to a ZIP file
-copying OTHER files of smaller, same, and larger sizes WORKS
What DOES NOT WORK?
-changing the name and/or extension does not work
-copying and pasting affected content into a text file that used to have different content and did copy before, then does not work
-Disabling SMB3 and SMB2 does not work
-modifying TCP auto-tuning does not work
-disabling WinFW on both client and server does not work
As noted above, if I modify the affected file to sanitize it's contents, it will work, so it's not much help.  I'm going to try to get a sample file exported that I can upload since I can't give you the original.
Your help is greatly appreciated!
Thanks.
Kevin

Answer 1

Hi Dharmesh,
Thanks for your reply!
The issue does seem to affect multiple users.  I'm not fully clear on whether it's multiple users and the same employee's file, but I suspect so.
The issue happens with a specific XML file and I've since determined that it seems to affect the exported RoE XML file for one employee (record?) in the software.  Other employees appear to work.
The biggest issue is that there's limited support from the vendor in this scenario.  Their app is supported on 2012 R2 RDS.
What I can't quite wrap my head around are
why does it work in RDP 7.1 but not 8.1?  What differences between the two for drive redirection would have it work in 7.1 and not 8.1?
when I examine the affected file, it really doesn't appear any different than one that works.  I used Notepad++ and it shows the encoding as the same and there doesn't appear to be any invalid characters in the affected file.  I wondered
if there was some string of characters that was being misinterpreted by RDP or some other operation and blocked somehow but besides having disabled AV and firewall software on both ends, I'm not sure what else I could change to test that further
Since it seems to affect only the one employee's XML file AND since modifying that file to change details in order to post it online would then make that file able to be copied, it seems I won't be able to post a sample.  Too bad.
Kevin