What is state-card-custom-ui.html for in my Lovelace default_view?

In Chrome JS console I can see the following error:

GET http://hassio.local:8123/local/custom_ui/state-card-custom-ui.html net::ERR_ABORTED 404 (Not Found)

It is because in default_view there is
<link rel="import" href="/local/custom_ui/state-card-custom-ui.html" async>

Any ideas what is this and why it’s there? Perhaps some custom component added that (as I didn’t)…
I think I can easily remove it as it’s not found anyway, but I don’t know what is the source file to edit…

Looks like you have an older version of custom_ui that is trying to interact with the DOM and is failing as it is not supported. Remove custom_ui

Could you tell me how? I checked Lovelace’s Raw config and can seen no signs of custom_ui…
And I have nothing to do with custom_ui in /www/

I never played much with custom_ui, but I’m pretty sure it would be in your configuration.yaml and would have been something you specifically added.

Spot on - didn’t check my configuration.yaml, it was in fronted: section.
Thanks a lot!