In the Configuration Panel, you might have chosen by mistake WorkflowGen authentication mode instead of IIS authentication mode and now you are stuck because there are no passwords set for any user.
To revert back to IIS authentication, go to the \wfgen\web.config
file and remove the following inside the system.webServer node:
<modules>
<add name="ApplicationSecurityAuthenticationModule" type="Advantys.Security.Http.AuthenticationModule" />
</modules>
This should set you back to IIS authentication mode