I'm having the same issue. Here are the error details:
Unhandled promise rejection: Error: fetch
'https://editor.construct.net/r350/components/bars/propertiesBar/propertiesBar.html' response returned 503 at FetchDocument
(https://editor.construct.net/r350/loader/htmlimports.js:2:153) at async Tpa
(https://editor.construct.net/r350/main.js:938:348) at async Promise.all (index 0)