Okay, I found a workaround. In ui/package.json, change
"proxy": "http://localhost:7575"
to
"proxy": "http://127.0.0.1:7575"
or
"proxy": "http://0.0.0.0:7575"
It seems others have come across this in create-react-app, which Daml-ui-template is based on. If the solution above doesn’t work for you, take a look at the other suggestions in the link above.