I was able to get it working again.
First I tried reverting to the previous version of the Arduino Nano ESP32 board, which made no difference.
Then I removed several libraries which I was no longer using, and reverted to the previous version of the ESP32Servo library which DID solve the problem.
I then updated the board back to the current version: still works.
Lastly I updated the ESP32Servo library to the current version: still works.
What I should have done is to remove one unused library at a time and not revert the ESP32Servo library at the same time, but I was getting frustrated and impatient!
My guess is that either something from one of the unused libraries was interfering in some way, or the ESP32Servo library become corrupted.
Anyway, I truly appreciate the help! I now understand how easy it is to manage versions of boards and libraries, so will have that in my basket of tools going forward.
Thanks much!