VNC conflict with system firewall

Hello,

i used VNC clients for years to connect to the Mac OS System for remote access. But since i upgraded to OS X 10.8 some of my VNC clients can't connect to the host system if its OS X firewall is turned on.

The connection works immediately if I swith the OS X firewall off.

I checked that the services aren't blocked but everything looks as it should as they're all marked green ("Incoming connections allowed").

I only use the system build-in VNC service.