Not sure about the issue in Thunder, but loading slrpnk.net in my browser is extremely slow on establishing TLS connections even before the anubis check comes up. If I had to guess, Thunder is either timing out connecting, or however anubis is configured on this instance, it's blocking access from Thunder.

Interesting that slrpnk.net comes up as one of the autofill options in the textbox when you start typing. But, as you say, Thunder still indicates that it "does not appear to be a valid instance" for some reason.

I assume that means that this Lemmy instance is hardcoded into Thunder as part of an a array of popular/common instances, but that something is "wrong" on the server side and slrpnk is not announcing itself in a way that Thunder is expecting.

Okay, so not just me then. Been having trouble accessing it recently as well, but never tested outside the app.