When I try to sign in to my Apple ID on my Apple Watch Series 2 (watchOS 4.2), I get an error stating that "An account with that identifier already exists".
The Watch App on my paired iPhone seems to think I haven't signed in on my Apple Watch before. (Apple Watch → General Apple ID → Sign In). However, my watch is already registered when I check the manage your Apple ID page.
What does this error mean? How do I fix it?