Still not sure the root cause of this. Per CERT_HAS_EXPIRED issue on SonarQube - Get help / SonarQube - SonarSource Community it was potentially related to a node JS version / issue.
Confirmed that quick node script performing a GET to the indicated /api/server/version endpoint works just fine on the machine where our hosted build agents are running. So that appears to be ruled out.
This is a blocker to our CI system at the moment. cc: @mickaelcaro since you ran into something similar before.