Citrix, PnaAuthDialog_popup window solution

If you get a blank window named PnaAuthDialog_popup when running Citrix receiver, the following solved this problem for me:

adding an extra line in /etc/sysctl.conf with:

net.ipv4.tcp_window_scaling=0

...and reboot.

This is taken from the comments at http://joepcremers.com/wordpress/connect-to-citrix-access-gateway-on-ubuntu-9-10/