In helping other users with this issue, we have discovered that on some routers UPnP doesn't always forward the correct ports that both Castle Crashers and PSN need to communicate.
This is why you might see UPnP "Available," and not be able to host properly. Please check the ports that are being forwarded in your router when your UPnP setting is enabled.
Castle Crashers operates on UDP port 3658. Sony uses the following ports to communicate between your PS3 and PSN: TCP Ports: 80, 443, 5223, and UDP Ports: 3478, 3479, and 3658.
If the ports listed above are NOT being forwarded by your UPnP settings, this is likely the cause. Please disable UPnP and manually forward those ports.