I posted about this earlier here: "Loading Failed" when soft crediting to a Personal Campaign Page
And Coleman kindly submitted a fix for it: https://github.com/civicrm/civicrm-core/pull/6156
Trouble is, I was never able to get the fix to work on my end and when I check the Demo site, which is now 4.6.6, it also still fails when you try to add a PCP page.
If I edit an existing donation from either a contact's card or by searching for a contribution and use the PCP drop down and try to search for a PCP -the screen below appears. I also tried opening the contribution in a new tab (rather than the overlay edit) and got the same error. I get the same error if I try to enter a check donation from scratch and add a PCP credit.
Did I miss something in the fix or is it not really working!?