XDMCP "maximum number of open sessions from your host reached" but no session is active

I'm using XDMCP to access CentOS 6 hosts from Cygwin/X. Everything works fine, but every now and then attempting to connect results in the error message: XDMCP fatal error: Session declined maximum number of open sessions from your host reached However no session is active and I was wondering if there was a way to clear these phantom connections so as to be able to connect again without having to raise the number of sessions allowed. I'm aware that this problem can be solved by rebooting the system, but sometimes that is not an option.