It took us 2 hours to figure this out and is basically a faceplant. Posting as answer to save the next person this issue as we didn't see a similar solution elsewhere
The Boomi instructions are quite clear yet in a spasm of epistemic arrogance, we refused to believe them
This is WRONG.
Instance url means
Instance Url Per the instructions
"In AtomSphere create a new HTTP Connection".
URL = Base URL of your Salesforce org's REST API. Salesforce instance
(e.g. na7) and version (e.g. v39.0) may differ. For example,
https://na7.salesforce.com/services/data/v39.0/. Remember the
remainder of the API endpoint will be configured in the Operation
You need to have:
na30 happened to be our pod; yours may be different.
Although the Boomi doc says it uses the SFDC OAuth web server flow, and that flow says that SFDC will return the instanceURL as per:
Boomi seems to ignore this and requires manual specification of the instance URL. As such, if your pod changes, your Boomi connection will break unless you coordinate the change.