After running the ISE Simulator from the PlanAhead tool, the ISE Simulator GUI is invoked and runs the simulation. Within the ISE Simulator GUI, the "Relaunch" feature exists to recompile the design and restart the simulation. When this feature is invoked, it relaunches the simulation without issue, but is causing the PlanAhead tool to be rendered unusable (it appears to hang).
To work around the issue, it is necessary to close down the ISE Simulator and re-run simulation from the PlanAhead tool directly.
This issue has been fixed in PlanAhead 14.5.