Happy to report our 900 works again. Sort of a shock because the steps taken today to get it running had all been tried before, carefully and repeatedly. One variable, which I don't know to be causal, is that today for the first time I used a fully charged new battery during the sequence described below. In routine operation, the 900 was behaving normally with the old battery, but I wondered whether the sync routine might require a higher level of power to the circuit board than my 5+ year old OEM battery could deliver. I replaced the OEM with an inexpensive new battery easily found on Amazon, and charged it overnight. Note that, regardless of the new battery, steps 1 through 4 as described below had never been necessary to sync this 900 before, but they are now, even though I'm using the exact same computer, USB cable, Harmony 900 and Harmony Remote Software as always. Operating systems and related software are obviously in a constant state of change, so it is what it is. Surprisingly, several of these steps are still not needed to sync our even older Harmony 520. The 900 is observably "fussier" now. Sort of a surprise that they behave differently, but they definitely do. I'm posting the sequence below in hopes they might help anyone else gets that gets stuck in a similar situation.
1. Disable Firewall.
2. Disable any browser Pop Up Blocker and Content Blocker.
3. Disable all antivirus software.
4. Put 900 in Safe Mode (by removing battery, and then reinserting battery while keeping the Off button pressed).
5. Sync using the Logitech Harmony Remote Software (LHRS) -- and if there are multiple USB ports on your computer, try each of them, even though all ports always work fine with all your other devices. My breakthrough came on a secondary port that I never use.
6. After taking steps 1 through 5 above, I was shocked to see the LHRS report 7% progress. Dozens of other recent attempts to sync had never reported any progress whatsoever. Encouraging for sure, but at 7% the LHRS stopped working, and a pop up warning appeared: "This update may not be intended for this remote control." Not true, but the LHRS aborted the download at that point. But at least there was communication between the remote and the software, which there had never been before despite taking all of the above steps on other unsuccessful attempts, including entering Safe mode. The new battery was the sole variable.
7. After trying twice in Safe mode, and getting the exact same result, I took the 900 out of safe mode, cabled up and started another sync attempt. Shockingly, it now completed a full sync.
The progress observed on this first full sync was unusual -- not smooth, definitely wonkier than normal. There was also some wonky content on the remote after this sync, including some activity errors. But after a couple of clean-up syncs, the remote was working perfectly again, and the observed sync progress returned to normal.
Steps 1 through 3 above are necessary now for a successful sync of our Harmony Model 900 remote, even though they are not needed for a successful sync of our Harmony Model 520. If you have a 900 that won't sync, hope this helps.
Here is a related support article