Initial guess is that you have put in the wrong credentials.
Second guess is that you have changed more that just the authentication pattern for the virtual proxy.
Do you use domain or local user accounts? Have you put in the current DOMAIN\USER format for the user name? Note, for local users you also have to use .\USER as the login format.
Try accessing QMC with your service account or the user used for making the server installation. If either of theses are local users remember to add .\ as the domain name.