Logs not visible until refresh, then other pages give error

I am running HA on Docker, and just upgraded to 0.115.2.
I noticed that when I go to the Logs page (server:8123/uilogs), I get a blank page.
This comes right if I hit F5 - the logs are then displayed.
But, then if I go to one of the other pages like the main lovelace page, I get " Error while loading page lovelace."
If I then hit F5 again, the lovelace page is correctly displayed (but now the log page will be blank when I go back to it).
Do I have something configured wrong?
I’m using HA behind a reverse proxy, and have the external and internal URLs configured (and can access from both inside and outside my firewall).

The only error I see in my log is

2020-09-21 19:13:53 ERROR (MainThread) [frontend.js.latest.202009180] http://server:8123/api/panel_custom/uilogs:20:11471 Uncaught NotSupportedError: Failed to execute 'define' on 'CustomElementRegistry': the name "mwc-list-item" has already been used with this registry

Any ideas?