Monitoring UI is blank after upgrading to 7.9.1

[ppf2]

Kibana version: 7.9.1

Browser version: Chrome Version 85.0.4183.83 (Official Build) (64-bit), Firefox 80.0.1 (64-bit)

Describe the bug:

A user reported an issue with the monitoring UI (showing blank) after upgrading from 7.7.1 to 7.9.1.

Browser dev tools console shows:

Kibana logs show the following warning:

{"type":"log","@timestamp":"2020-09-09T19:41:38Z","tags":["warning","plugins","alerts","plugins","alerting"],"pid":25609,"message":"APIs are disabled due to the Encrypted Saved Objects plugin using an ephemeral encryption key. Please set xpack.encryptedSavedObjects.encryptionKey in kibana.yml."}

After setting xpack.encryptedSavedObjects.encryptionKey to a concrete value of 32 or more characters (of anything), the monitoring UI started to work.

I cannot locate any documentation on a breaking change related to the requirement of setting xpack.encryptedSavedObjects.encryptionKey for the monitoring UI to work between 7.7 and 7.9 releases. It will also be helpful for the monitoring UI to catch this condition and return an intuitive error message to the page :)

[elasticmachine]

Pinging @elastic/stack-monitoring (Team:Monitoring)