Splunk has been problematic before through the IVE rewrite engine so this isn't surprising.
Splunk 6.0 gets stuck at "loading" the dashboard on IVE 7.4R5 with custom headers enabled (which fixed splunk issues prior to 6.0)
Realize Splunk 6.0 just came out but just letting you guys know about it. I haven't tested it with passthrough proxy yet.
Thank you for the information. I have not seen any reported issues for Splunk 6.0, but it looks like Splunk made a change which requires a change in the rewrite engine to resolve the issue.
Please open a JTAC case and provide JSAM access to the Splunk resource so JTAC can replicate the issue.