Please put yourself in our position. We're sitting at home or at work reading your comments, but we didn't set up your environment. We don't know anything about your pc's, your accounts, your network, nothing.
So when you comment "Out of the blue we get the fun organization’s security policies message", we have no idea what that message says. We have no idea what circumstance would cause that message to be displayed.
When you say "Was fine last week, this week it's a no go", well, what did you change? We don't know what you or anyone else in your organization has done.
Who manages your network? Have you asked them for help? What do they say?
Are you using Active Directory or local accounts? I do not recommend using the guest account. If you are using local accounts, then I think that it's better to define accounts with the same name and password on each PC so that you can cross authenticate. Credential manager is another alternative.
Based on "Still won't connect.", please describe what account you are using. Is there an equivalent account on the target machine? What error do you see in the security eventlog on the target machine?
We're happy to help you, but we really need specific details about what you are doing.