I was playing in my SCCM 1511 test environement and started to notice that my MP is not working anymore.
mpcontrol.log said that there is error 503 "Service Unavailable".
I reinstalled MP, but that didn't help. I reinstalled IIS and MP and that also didn't help.
Finally I found out here https://technet.microsoft.com/en-us/library/mt589738.aspx that MP needs to have .NET Framework 4.5.2, but I had only .NET 4.5.
After installing .NET 4.5.2, errors disappeared in mpcontrol.log and MP started to work.
Hope this helps!