I am unable to connect to clients - receiving Connection Lost error

I cannot connect to any clients via zoho assist. These are clients who have been set up for months for unattended remote access.

I've used updated version of both Chrome and Explorer, but always receive the following error:

Connection Lost! Reconnection attempt under progress...

I've tried uninstalling, then reinstalling, which was unsuccessful.

Some of these clients are behind firewalls, some are not.

Zoho Desk Resources

Sticky Posts
Release Notes: Version 1.91, February 2015
Unattended Access (Beta) is live! Unattended Access, a feature requested by a lot of our customers is live now. Currently, it's in beta. You can add up to 100 computers per Technician subscription. They can also be accessed by your organization members. My Computers list in Zoho Assist home page will always display the online status of your unattended computers. You can rename the computers whenever you wish. Currently, Unattended Access can be configured only for Windows computers. Nevertheless,