Hi John!
Yes, you are correct, it is a custom, required field with a "Text" attribute. I went through and tried it again and noticed on the second story, all 4 of fields that are of the "Text" attribute type are not enabled! This includes the Description field and three custom fields. The only way to continue is to close the session/browser and do one story at a time.
Scenario:
CAAC/Rally On Prem version 2016.1
Firefox Browser 31.5 ESR
Using New UI, creating second user story, "Text" attribute fields are not enabled.
Are you able to verify/replicate this scenario to see if it is an application bug?
Thanks for your assistance!