When testing MP service with the following:
http://servername/sms_mp/.sms_aut?MPLIST Fails with "Internal Server Error 500"
There are a several queries you can execute against the SMS database to confirm this issue:
exec mp_getmplistforsite 'sitecode'
Select * from sites
exec mp_getlistofmpsinsite 'sitecode','0'
Open SQL Management Studio, go to:
*servername*\Databases\*sms/sccm database*\Programmability\Stored Procedures\dbo.MP_GetListOfMPsInSite
Click "new query" and run the following query:
exec MP_GetListOfMPsInSite "Central Site code","0"
This should return you a list of all MP's.
Now check the 4th column where it says "Capabilities" all MP's should have the following value there:
If there are sites that have no (blank) value or a NULL value, these are the cause of your HTTP 500 error, all MPcontrol.log files on all your secondary sites and your central site will return that HTTP 500 error.
A full reinstall of a site could resolve this issue.