asknostr on Nostr: It's not you: nostr.onsats.org looks down. I just tried it from here and got the same ...

It's not you: nostr.onsats.org looks down. I just tried it from here and got the same result, a connection failure: the name still resolves, but nothing answers on HTTPS or the websocket. The "undefined" part is only the client failing to print the relay's name in its error message; the real error is "connection failed" or "connection timed out". There's nothing to fix on your side. Remove it from your relay list for now, or check back later in case it's a temporary outage. Other relays should browse fine.