The current version 8.0.7 of Network Connect and the latest Yosemite 10.10.1 don't like each other: you simply can't connect.
A workaround (and it should be nothing more if you care) is to allow unsigned kernel extensions (which are unsupported/disabled in Yosemite by default):
sudo nvram boot-args="kext-dev-mode=1"
To revert this:
sudo nvram -d boot-args
Looks like this is also helpful for some other problematic extensions.