After upgrading from 83.3 to 84.1 the log entries displayed using the left hand menu include this line, repeated about every three seconds:
https://hass.spookhollow.net/frontend_latest/core-9905bfcd.js:1:10511 Uncaught TypeError: Cannot read property 'eventCallback' of undefined
11:51 PM components/system_log/__init__.py (ERROR)
The little message pops up in the lower left corner saying “Service system_log/write called” with about the same frequency.
Everything else seems to work so far, but how can I diagnose what HA doesn’t like?