I've found the message "Portal Web Site Control Manager detected PORTALWEB is not responding to HTTP requests. The http status code and text is 401, Unauthorized." in the logs of the server which hosts our SCCM 2012 Application Catalog, when diagnosing the issues with the 'SCCM Software Center' failing to display available software showing a message of "Loading Software Center returned error code 0x80041001(-2147217407)". Also when browsing to the Application Catalog it displays a "Server Error in '/' Application" error. Googling the issue, it appears to be widespread with SCCM 2012 and related to WCF Activation.
The resolution can be found in the following MS KB - http://support.microsoft.com/kb/2015129
I would also recommend testing any Application Catalog related issues experienced on a client by accessing the Catalog via other clients. I have found clients can be at fault more times than the server.