In El Capitan, Simon can not identify the registered Twitter account.
Yes, this is a known OS bug, one of the ones mentioned in my recent "Apple's neglected OS" blog post. It's a very annoying bug, where Simon's attempt to authorize access to Twitter accounts fails. There's not much I can do to fix this, since it's an OS bug with no workaround that I'm aware of.
I actually have figured out a workaround for this, which I'll blog about soon, and will include in the next Simon release. I'm not sure yet if that will be 4.2.1 or 4.3.
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