In the Facebook forum, Dan Kulp responded that he found my TreeStar model had "extra" node levels that were crashing v51. The TreeStar only has 4 nodes (4 strings of white, LED Rope Light), but inside my problem sequence, it was showing 18 nodes when you doubleclick to expand the node listing.
Since I could open the file in v50, I went back to it and looked. There were many seemingly random but not-editable effects on Nodes 5 thru 18, but when I scrolled across the timeline, there were also some editable (delete-able) effects in those Nodes. Once I deleted all of those mystery effects, saved, exited and reopened, the Nodes 5 thru 18 were gone.
Then I reinstalled v51 and the sequence opens just fine.
Still waiting to know just how Dan found that problem so accurately. He also said that it should NOT have crashed xlights. (but it sure did).