Mac Pro (Late 2013) Freezes When Connected to LG UltraFine 5K

I have a Mac Pro (Late 2013) connected via a Thunderbolt 2 cable to an Apple Thunderbolt 2 / 3 adaptor plugged in to an LG UltraFine 5K display.

This configuration has worked well for over five years. (I used to experience an occasional flicker in which the screen would go black for a couple of seconds, but I think that was linked to RF interference from a power strip that was close to the display, and it hasn’t reoccurred since I rearranged my desk to move the power strip further away.)

About a week ago, I upgraded to MacOS Catalina, and yesterday I purchased a subscription to Microsoft Word; these changes may be unrelated to my problem but I mention them because they’re the only configuration changes I made recently.

Yesterday I rebooted my computer and after it restarted the screen stayed blank. 

I tried a few different things, but none of those fixed the problem:

• unplugging the computer’s power cable and waiting a minute before reconnecting in order to reset the SMC.

• starting up with the command-option-p-r keys held down and waiting until I heard the second chime indicating that the PRAM had been zapped.

• unplugging and reseating all of the video cables.

• swapping in a different Thunderbolt cable.

• switching to different Thunderbolt ports on the Mac.

• went out and purchased a new Apple Thunderbolt 2 / 3 adaptor.

I disconnected the LG 5K display, and borrowed an older display with an HDMI cable, which worked fine — computer booted, display lit, all of my files are still there, etc — so at least superficially the computer itself is okay. When I run the System Report and click on Thunderbolt it shows all three busses and doesn’t show any obvious problems with them.

However, when I reconnected the Thunderbolt cable linking the LG 5K display to the Mac, I get one of the following behaviors:

• If I plug it into the Mac’s first Thunderbolt bus (ports 1 or 3), the LG 5K display remains black, the HDMI display remains active, but after a couple of seconds the computer stops responding to keyboard input; mouse movement is reflected by the cursor on the screen, but clicks have no effect and things that would normally respond to the mouse hovering over them do not do so.

• If I plug it into Thunderbolt bus 2 (ports 2 and 4) or bus 3 (ports 5 and 6), the LG 5K display remains black and after a couple of seconds the HDMI display reports loss of signal and goes black as well.

When I disconnect the Thunderbolt cable, after a couple of seconds the problem goes away — if the HDMI display was off it comes back on, and the keyboard and mouse start working again.

However, if I’ve left the Thunderbolt cable connected for more than about ten seconds, when the computer recovers, all apps have been abruptly closed and I find myself back at the login screen.

I considered trying to reset / upgrade the firmware on the LG, but I can’t usefully connect it to the Mac to do so.

Does this condition sound familiar to anyone?

Any advice about how to isolate the problem to either the Mac or the LG 5K display?

Thank you for any guidance you might have.