Ok, scrub that - the problem in my case was that I was using the 14.0 databricks runtime, which appears to have a bug relating to abfss paths here. Switching back to the 13.3 LTS release resolved it for me. So if you're in the same boat finding abfss paths in kafka.ssl.keystore.location and kafka.ssl.truststore.location are failing, try switching back to 13.3 LTS.