Just to be clear, as J-NIC wrote in a comment, adding additional classes to the unarchiver is what worked for me. In my case I just needed to add NSString: let mySpecialObject = try? NSKeyedUnarchiver.unarchivedObject(ofClasses:[MySpecialClass.self, NSString.self] ...

I just got this error and I'm on Version 13.2 (13C90)

So in the end it turned out that "Local Network" in Settings was off. But everything else the app does on the local network was operational (HTTP, etc). We were experimenting a while ago with techniques to try and detect if that setting was on or off (it does not appear there is any straightforward way to make that determination), and I must have left the toggle off. Sigh. But Quinn, thanks for the link above - it was very enlightening (even though we seem to have had everything defined properly).

Possibly related - when I connect to the camera in Settings, after it connects I get a yellow "warning" below the network name saying "No Internet Connection". This does not happen on my iPhone 6s running iOS 14.2