In my case, I have made NO changes of any kind to my Verizon Account details since activating the cell number in my Verizon Messages March 2016 with a new Samsung Galaxy S7 Edge.
I replaced the S7 Edge (battery was bulging) with my current S9+ March 2018.
All 5 other "Devices registered with integrated messaging service" (see attached snapshot below) have worked smoothly (with only a few glitches) over all the intervening years.
gts4lwifi = Samsung Galaxy Tab S4
- working since April 2019 - continues to sync correctly today
JohnF iPad = Apple iPad Air
- working since July 2016 - continues to sync correctly today
Only all 3 Windows based Messages apps (2 different versions) stopped working.
I've done some more checking and Verizon Messages stopped working, seemingly NOT corresponding to any Windows Updates:
P6T-DELUXE-V2 - last sync 7/19 - OS was at Windows 7 Pro SP-1 64-bit (6.1.7601)
- before latest update 7/21/2019 (KB4507437)
P9X79 - last sync 7/19 - OS was at Version 1809 (OS Build 17763.615)
Dell_XPS_15_JAF - last sync 7/17 - OS was at Version 1903 (OS Build 18362.239)
I'm changing my "theory" to it being something Verizon changed somewhere in their system.
Any other theories? Can we dig up any more details?