Could it be the address (1N8vh...) I copied is not, in fact, the address but the name of the wallet?
Unlikely. The default wallet is named
default_wallet, and that's not something you just turn into a Bitcoin address.
One possibility is
clipboard malware, although I haven't read about malware that replaces a bc1q-address with a legacy ("1") address.
If this is what happened I suppose this is a fatal error on my part and, given the security protocols, the bc can not be moved out of 1N8vh... since there is no such address owner.
It's unlikely you create an address by just typing it (I think there's about a 1 in 4 billion chance to type a correct checksum), so it's much more likely the address is somehow correct.
Your "fatal error" is not checking your wallet after the first withdrawal. Checking block explorers isn't enough if the address doesn't belong to your wallet.