The first thing to check is that the licence key has been applied properly. Check this in the QMC.
Check also that all services are up and running correctly.
There may be some extra information in the Windows event viewers.
Check that the user account used for the service and the user account accessing AccessPoint has all the required permissions.
A restart can sometimes cause things to start working better also.
Hope that helps,
This issue happens when you are not getting directed to the Authentication page and trying to use the cookie from the browser, if the App Pool is not set with the right .NET Version or the .Net Version is not Registered properly on the machine.
Please troubleshoot the issue as per the following site and can perform the workaround mentioned.