This is a default install for both builder and server.
cfadmin url is: http://wcistest:8500/CFIDE/administrator/index.cfm
RDS config is: 03.15.2019-09.08.33
Following your clue regarding cfadmin url, I changed the port in rds config to 8500 and it worked. Is the admin port in the cfserver install randomly selected? If not, then the cfbuilder install needs to be updated to use 8500. If it is, then some kind of dialog in the cfbuilder install or start page informing one of the need to change that port might save some other unfortunate soul a lot of time!
Thanks for your help!