Which version of the Firewall and Administration GUI are you using?
I don't recall ever seeing this error myself. The fact that you are able to log into the CLI is good news and suggests that there isn't anything fundamentally wrong with the Firewall.
If I were to guess I'd say that it might be a mismatch between GUI & Firewall (version 7 GUI and version 8 Firewall).
Can you upload a screenshot of the error?
The firewal version is 7.0.1.02 and admin console is 4.10.
Have other firewalls the same version and able to get on them no problem have also tried diff laptops to role out client end.
Yes, I agree those version numbers look OK. My v7 console is running at 4.11, but that's because I've connected to a Firewall running 7.0.1.03 and it has automatically upgraded.
I can only think that, for some reason, the certificate used to encrypt the admin console session has got itself mangled in some way. Try a Firewall reboot (when you can) and if that doesn't make any difference you may need to raise a service request with McAfee support.
Have tried several reboots, changing time/date on firewall and PC. Nothing seems to work.
Only option is to flaten and start again, unless there are any other suggestions?
I'd certainly suggest raising the matter with McAfee support before resorting to a re-build.
Can you upload a screenshot of the error you are seeing?
While I've not seen it before, maybe the context in which it is appearing may shed some light.