Fortigate explicit webproxy chaining problem

Hello everyone, i have a problem with the proxy chaining feature and explicit webproxy. I use Kerberosauthentication with explicit proxy on two fortigates 2000e and 500e. Now i want to redirect one url from the explicit webproxy from the 2000e to the 500e with the web proxy forwarding servers feature. The forwarding works fine and i have a proxyrule at the 500e that allows all to reach the forwarded address without the need to be in a special group or authentication. But when the forward from the url from the 2000e explicit proxy takes place then i get the replacementmessage authorization failed. Can anybody help me ?

Thank you