I believe you are encountering a known challenge - NIM061853; Mobile Project Center does not work with proxy server that requires authentication. Please contact Esri Support and have your information associated to NIM061853 and then you will be able to track the status of this NIM via My Support option on Esri's Support site.
My local ESRI support confirms there are known bugs: #NIM061853 and #NIM063099. I hope those bugs will be resolved soon cause they are critical in all our telco sites.