If it shows error code 526 in the box on the ARC settings page, this is a known issue. ARC works anyway, there’s just the test error. It’s part of the cloud issues that are announced in the big blue banner on top of the forum.
See Roon comment here:
Fixed link, I meant this:
(@vadim I wonder if the new Discourse forum version has a bug because I posted many links yesterday and I don’t think they were all wrong, but today many of them pointed to entirely incorrect posts)