TV mediagateway cannot access Win8.1 shared folder

Hi all,
I'm struggling with this problem from two weeks. I read many similar problems in this forum, but none of the proposed solutions worked with me.

PROBLEM
I have two TV mediagateways (Memup Mediagate Lnx HD) that read files from a LAN folder shared by a Windows 7 PC. It worked fine for many years. After the clean installation of Win 8.1 Pro, the mediagateways are no more able to "login" into the PC to read the shared folder.
Similarly to Win7, they recognize the domain (WORKGROUP) and the PC name (DESKTOP-SERVER); when I try to enter, they correctly ask me login and password. The difference is that now they return "LOGON ERROR" without other infos.

CONFIGURATION
In the same network and domain, I have a laptop with Linux Mint, a Canon MG8150 printer and an Android smartphone. Doing some test:

• Mediagateways are able to read folders shared by Linux Mint and by the Canon printer.
• Linux Mint and Android are able to read the Win 8.1 shared folders.

So, everyone speaks with everyone else, except the couple mediagateway <-> Win 8.1

In order to make the system as simple as possible:

• All the Win 8.1 accounts are local and there is not the Home Group
• Antivirus and firewall are disabled
• there is only one domain (WORKGROUP)
• the shared folders are within the internal HDD (not USB).
• Everyone under Advanced Sharing has Read Access; Everyone under Security has Read Access, List Contents and Read/Execute
• Disabled password protected sharing
• Enabled cryptography at 40/56 bit

ATTEMPTS
Until now I have tried to:

1. disable SMB2/SMB3
2. change IRPStackSize until decimal 50
3. change Network security: LAN Manager authentication level
   Set to Send LM & NTLM responses only
   Set the Minimum session security for NTLM SSP
   Disable Require 128-bit encryption

I have no clue of what is happening between mediagateway and Win 8.1 because it is not logged in the Event log.
Do you know how I can have more info about the error, with event log or with a third party application?

I have run out of ideas. May you please help me?

Thanks in advance
Johann