AFAICT, you didn’t do anything wrong. This error means that there’s a problem with bors, which is apparently trying to use an access token incorrectly. GitHub Apps have a root credential (the “private key”) which they use to make signed requests for repository-specific access tokens. The private key rarely changes, while the access tokens are only supposed to last for an hour. Either it failed to expire it, or it simply tried to use one that belong to a different repository.
I restarted it, thus clearing the cache entirely. Obviously, this is just a workaround, but I’m not sure what the proper fix is. If I see the same error happen more often, I’ll be better able to diagnose it.