Remote Management Port 5900 Not Responding
I did two things in sequence, which cleared up the problem.  First,

sudo /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/kickstart -activate -configure -access -on -users admin -privs -all -restart -agent -menu


Second, in the GUI, I turned off remote management and turned on screen sharing.  now port 5900 responded.  Next I turned off screen sharing again and turned on remote management.  Now it is still responding.

Probably a bug in macos...