This will happen if the fw Server is on v6.4 or later versions v6.5 or v6.6, etc. and the fw Client desktop application is running on a lower or no-longer "auto-upgrade" supported version.
Solution:
You will need to figure out which user is trying to connect from which machine which has an old, below version ... and not yet have a manually upgraded fw Client application installed.
Here the link to the installers files on our website: Public Installers
This can also be the result of fw Server app Ports being open and exposed to the internet. And these “Non-Fatal Error” messages are the result of potentially malicious connection attempts, which get deflected from the fw Server app.
For increased protection of your farmerswife environment please take a look here:
Using NGINX as a third party proxy service to apply your SSL certificates