UPDATE: I attempted now a high concurrency cluster with runtime 10.1ML.

I don't have time atm to investigate all options but I am guessing that the spark-ui is simple not working on standalone single node clusters or it was the specific (9.1LTS runtime but I doubt that).

It is not that relevant on single node clusters anyway.

I have a high concurrency 10.1ML cluster running and this one show all the ui everywhere.