Hello HomerJ,
I have experienced this issue once in the past. the first things I would recommend are the following:
apparently I stumbled across a reference that says the issue wuth tls1.2 and SSL for excel was resolved as of patch 2? If you are not using at least patch 2 it is necessary for you to upgrade to patch 2 so that the issue may be resolved within the code as to how TLS 1.2 is handled accordingly.

Also are you using an older version of the add-in or the latest offering from the marketplace as the older ones had an issue with ssl and required it to be uninstalled, the system rebooted (specifically says the system must be rebooted or it still may interfere with the installation and operation of the new one), and then the new add in installed (and only one version of the add-in is allowed at any given time)?

If this is not the cause of your issue then you will need to look into certificates and makes sure that you have the latest signed certificates as recently they have changed and this is causing issue with the manufacturer name on the certificate and its interaction with the client software and that on the clients system. It must also be accepted within the certificate information settings within Excel, which your IT is responsible for supporting you with such actions.

I hope this helps with your issue and if not I would suggest you open a support case with your support group to address your specific issues.

Dan