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.