I have upgraded to 6.5.1 and am having similar issues. When I get to Setup a show, all of my dashboards show up, but after I choose the dashboards to cycle through and click on Start Show, I just get a blank screen.
I have cleared my cache in both IE and Chrome, used the /en-US/_bump and restarted Splunk, but still am getting the same results.
Errors from the browser are
IPT5009: 'is_absolute_url' is undefined File: SlideshowSetupView.js, Line: 6, Column: 11638
Thanks for the report. I'm going to try to get a local repro soon and cut a new build accordingly.
That fixed the issue, thanks Luke for the awesome support.