I have the exact same issue as Starchild - sending a custom variable with the “segment” query parameter to the dashboard makes all widgets empty. I just updated from 1.8.2 to 1.9.1. Before the update, the segment had no effect at all on the dashboard.
I am 100% certain that there is data for the segment, because if I use the exact same segment for a single widget then the data shows up as expected, filtered as it should be. The same widget is empty in the dashboard when the segment is used there.
I did some additional testing, and I believe that the dashboard doesn’t work when more than one segment is used (passing a custom variable requires two segments). I posted a bug report in the tracker here:
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