I understand it's gone now, but you could have used \errverbose to display the source function name that caused the error, followed by setting backtrace_functions to that function and repeated the query. That may have resulted in a more precise location of the problem.
It is not entirely gone, I still have some indexes around that produce this. This backtracing functionality is available only in the HEAD as far as I can see, so it took me a while to get to it: