Users are able to connect, but when the login screen appears and credentials are keyed in, it reappears with an empty password field. I presume that means the credentials are not accepted thus interrupting the login process.
It means that the access password is either not set or incorrectly typed in.
To set/change the access password, access the Host settings, click Security -> Change password and set/change the access password.
Hope that helps.
P.S. Sorry for the Windows upgrade issue. This is a known bug and it happens because the Host's settings are located in the HKEY_LOCAL_MACHINE/SYSTEM folder which is wiped by Windows during upgrade (hence, the Host settings are reset). Starting the very next update (v. 6.5) the settings will be located in another place in the registry so this problem will be no more.
We didn't receive any reports recently about the antivirus programs that you mentioned treating our Host badly. We also do regular checks on our own and monitor antivirus activity as it applies to Remote Utilities. There was nothing alarming.
Still, what you describe looks very much like antivirus software activity. Please, do report to us if you find a consistent pattern.