Thanks for the response. I checked the configurations and limitations in the reference doc you supplied and all is good as far as I can see.
We are on 19c. We do not have Oracle Text Search enabled, so no CTX_DDL package was installed. We are not in a non-English locale. There are no double quotes in any field names. Our Oracle JDBC driver was installed correctly and during installation, we used the GLOBAL_DBNAME, not SID
I did run a debug on my collaborator when the error occurred, and opened a support ticket attaching the debug output.
One of the lines in the error log is intriguing:
SEVERE: Last error captured before entering debug mode:
com.google.gwt.event.shared.UmbrellaException: Exception caught: Exception caught: (TypeError) : Cannot read properties of null (reading 'Vc')
I am waiting on customer support to respond, and if it proves helpful, I will post a solution here if none is provided by other users.