Sorry to hear you are having issues. Does the issue happen with a specific file or with any file (e.g. https://petstore.swagger.io/v2/swagger.yaml)? If this happens with a specific file, could you please attach it here or PM it to me?
Also, which versions of Chrome and Ubuntu do you use?
Have't tested others.
Since that is public url, I believe most of the world might be accessing it and might have reported by some one.
Have you tried any file or url and it worked on the same site (editor.swagger.io)?
The issue might be Ubuntu-specific - which versions of Ubuntu and Chrome do you use? Could you please check if there are any errors on the Console tab in Chrome Developer Tools?
I'm asking because file import works fine for me, but I'm using Chrome on Windows 7.
I can only think of 2 cases where "Import URL" (not "Import File") may not work:
Both cases are caused by browser security restrictions, and the workaround is to save the file locally, and use the "File > Import File" instead of "File > Import URL".