Setting disableUploadTimeout="false" has resolved the problem. Whoo hoo!
Thanks, PTC Support.
I still maintain even after careful and repeated reading of the Tomcat
documentation that changing the setting from the default "true" to "false"
seems counter-intuitive as far as creating a connection tolerant of latency
/ dropped packets / jitter ... whatever the specific problem was that we
were having with our WAN to India. But, it's a good day when you can close
a ticket.