It's a JSON parsing error.  I don't know if it's a known error, but it's
not one I've seen chatter about on this list.  I've gotten it too and
haven't really looked into it.  I suspect it is a bug in the Emacs JSON
parsing function. but that I can only support that suspicion with
hunches and slander.  

Just to round out my "me too" report: I sometimes get that error on
opening a thread from the search view.  It seems to be intermittent in
that a thread will open just fine and then I'll select it again and I'll
get the error.  But that observation might be faulty memory.

Best regards,
James