I have the exact same error, I'll try to do what you had which was using 7.6.1 to see if it will start working again. In my case I am running behind a proxy so I set the following environment variables
- "SERVER_BASEPATH=/kibana"
- "SERVER_REWRITEBASEPATH=true"
- "LOGGING_SILENT=true"
Unfortunately 7.6.1 didn't help either. BTW I didn't get anything on the logs that appear relevant even with LOGGING_SILENT not set
What worked in the end was downgrading Kibana to 7.5.2