Ok, I think I found the problem:
LivyHttpRequestFailure: Something went wrong while processing your request. Please try again later. HTTP status code: 500.
This morning I tried to start up a Databricks session, but I got an error:
LivyHttpRequestFailure: Something went wrong while processing your request. Please try again later. HTTP status code: 500. Trace ID: efae68a9-69fc-4886-9283-9b795f16b4ea.
When I search on LivyHttpRequestFailure, I get exactly 1 result and it doesn't address my situation, since it even couldn't start the session. I also tried with other notebooks, and got the same result.
Looking at the above, I got the thought it might be an Azure problem, but on Azure Status everything looks green.
Am I missing something?