After a recent in-place server OS upgrade, I thought everything was running smoothly but noticed that in the MPControl.log on the site server I was getting the following message
Call to HttpSendRequestSync failed for port 80 with status code 500, text: Internal Server Error

I wasn’t sure if this was related to the server OS upgrade or not but thought it was worth blogging the fix regardless.
I ran the MPList URL in Edge browser (take a look at Garth Jones’ blog post on this) . For my Management Point I entered the URL http://configmgr.internal.sccmsolutions.com/sms_mp/.sms_aut?mplist
The result was a 503 error and digging into the details on the resultant page I made a note of the following:
CONFIGURATION FILE IS NOT WELL-FORMED XML & the path c:\inetpub\temp\apppools\DefaultAppPool\DefaultAppPool.config
Opening the apppool.config file revealed the following information

So a corrupt application.config file was to blame. This file resides in the location %WINDIR%\System32\inetsrv\Config. Nothing looked amiss, however I decided to back up the file.
In C:\inetpub\history (or wherever the inetpub folder is located on your site server) is a backup of the configuration files. I took the most recent file and copied that over, then ran an IISRESET and the management point reported back as healthy.

