It may be possible that IIS on the MP is broken due to that HttpSendRequestSync error. Did you add/remove any CM roles or software just before this started happening? Here are a couple of links on troubleshooting it.
- https://sccmentor.com/2017/06/27/call-to-httpsendrequestsync-failed-for-port-80-with-status-code-500-text-internal-server-error-again/
- https://social.technet.microsoft.com/Forums/ie/en-US/66f3f171-0303-4007-b8b0-9a78a43e165a/solution-call-to-httpsendrequestsync-failed-for-port-80-with-status-code-500-text-internal?forum=ConfigMgrDeployment