I've been working with John Melas on this issue for the past 2 weeks and so far nothing has helped. He suggested I turn to this forum for help.
Any suggestions re what I could try (beyond what I've already tried) are much appreciated.
Also, if you've seen or heard of this kind of issue happening with any version of John's tools it would be helpful.

Here's the details of the issue and what I've tried so far:

Total Librarian (part of Montage Tools) hangs up whenever I do "Receive All" or "Transmit All".
The only way to un-hang the Librarian is to kill the process in Windows Task Manager and restart it.
Another issue is that the Performance Editor doesn't start up properly ... I see the initial banner come up for a few seconds, but the main window never appears ... although there is an icon for the Performance Editor at the bottom of the screen.

I'm running on a PC with Intel i7 processor and Windows 10 Home edition Version 1903.
Montage is running Firmware Version 3.00.1.
Total Librarian is version 1.50
I am using the latest Yamaha Steinberg USB Driver (V1.10.4 for Windows)

I set MIDI-related settings to the following (as directed by John Melas):
- The MIDI settings in Total Librarian are: "Montage-1" as MIDI in and out and "none" for Auxiliary Ports .
- Montage Midi is set to USB and the settings in the MIDI section of Utility->Settings->Advanced are:
* Device Number = All
* Bank Select = ON
* Program Change = ON

It is clear that there is communication between Total Librarian and Montage as evidenced by:
- Librarian indicates "Montage 8 v3.00.1 connected" at the bottom of the screen.
- I opened a Library into Total Librarian that I previously saved on my PC and then did "Send to Edit Buffer" and "Receive from Edit Buffer" ... these both worked!
- I then setup MIDI Ox (MIDI monitoring utility S/W) to monitor the messages between Total Librarian and the Montage during the following conditions :
> Upon startup of Total Librarian I see several SYSEX msgs exchanged (see attached "Montage Tools MIDI msgs upon startup.JPG)
> Upon "Send to Edit Buffer" I see 2 SYSEX msgs exchanged, 1 in each direction. (see attached Montage Tools MIDI msgs upon send to edit buffer.JPG)
> Upon "Receive All" command .. .in this case I see no SYSEX msgs exchanged ... or any other MIDI messages.

Also, Montage Midi via USB still works ... at least as far as Note on /Note off messages go.

I also tried the following ...none of which helped:
- I reset the Montage to Factory settings (using "Initialize All Data")
- I tried uninstalling and re-installing Total Librarian version 1.50 and Yamaha Steinberg USB Driver version 1.10.4.
- I tried using "Revo Uninstaller: to uninstall all of John Melas SW for both Motif and Montage and delete any remaining Registry entries related to these programs. I then re-installed Montage Tools version 1.50
- I tried using "Revo Uninstaller: to uninstall all of Yamaha Steinberg USB Driver and delete any remaining Registry entries related to this program. I then re-installed Yamaha Steinberg USB Driver
- I tried using DIN MIDI In/Out cables (with appropriate MIDI Settings) instead of MIDI via USB
- I tried turning off or disabling all other MIDI devices in my system
- I tried disabling all "Start up" processes in my system (i.e. background utility processes)
- I tried using the previous version of Montage Tools (v 1.4.0) and also tried Montage Tools version 1.20 ... which I think was the last version Montage Tools that I had installed which worked (at least "Receive All" didn't hang up) a few weeks ago. Version 1.20 now has same issues I see in version 1.40 and 1.50.

So, based on the last bullet above, it is clear that the issue is not something unique to Montage Tools 1.50. The problem is apparently something other than Montage Tools... but what??