I got this response back from Avid FYI:
"I think you are running into a known issue here (PT 217940)The summary of this issue is: Pro Tools 11-12 under Windows 8-10 does not work properly with MOTU MIDI interfaces.This issue is specific to MOTU's MIDI interfaces that offer stand-alone capabilities (ie routing matrix, MTC/LTC sync etc….) In their current product line this includes the MIDI Express XT and the Micro Express."
I also found out that some MIDI interfaces come without drivers: They are called "Class Compliant" -- Sweetwater's website has this "word of wisdom" defining "Class Compliant."
In common usage, this means that a
USB peripheral device is “
plug and play,” meaning you can plug it in and it will work a Mac or
Windows computer without installing proprietary
drivers.
Mac and Windows come with generic USB drivers; there are several types, known as “classes,” such as USB Video Device Class, USB
MIDI Device Class, USB Mass Storage Class, etc. Peripherals that can work with those class drivers are referred to as “class compliant.” Because those “class” drivers are generic, they may not support product-specific or proprietary features — to access those non-“class” features usually requires a proprietary driver from the device’s manufacturer.
Hopefully the new MIDI interface will work.
And it worked!!