I used to use this client and had some strange issues, and thankfully no longer need to (famous last words ?).
Do you have any other VPN clients loded ? The Contivity was very fussy about this, and I always found the most stable opton was to load Contivity first, and then load the others after (Cisco and an other). When removing these to get it back to a ‘clean’ state, you may need to go through the register and remove any mention of all clients loaded, including things like ‘deterministic’ which is used by Cisco and others.
If that doesn’t fix the issue, then you could try removing any process relating to the VPN client, and/or disabling the network interface and re-enabling it again. Not ideal, but if they save a reboot, then it should be quicker.
Your best bet is for the customers to come to their senses and get rid of the Nortel for something else