After my machine installed KB4556799 last night, Tftpd64 would no longer open. I received an error message "could not connect to service as machine actively refused it" or something like that. (my apologies, I didn't document exact wording).
This is a cumulative update for Win10 v1909 that was installed automatically.
Uninstalling this update corrected the issue and it is working as expected. This is the only update I needed to uninstall.