While accessing the MDM  application through Apache LB server we may get below exceptions with long running threads/Process.
 

Error:-Proxy Error
The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request POST /eml/AJAXRequestInterceptor.

Reason: Error reading from remote server

During MDM URL access and some specific operations we may get exception as The proxy server could not handle the request POST /eml/AJAXRequestInterceptor

ALL

We will encounter this error due to Proxy Default timeout,Which is getting exceeded during a long running thread.

So,Increasing the Proxy timeout at Apache level will resolve the issue.

The following configuration will process all files in the /www/data/ directory for server-side includes.

<Directory "/www/data/">
  ExtFilterDefine INCLUDES
</Directory>

TimeOut Directive:

Description:    Amount of time the server will wait for certain events before failing a request
Syntax:    TimeOut seconds
Default:    TimeOut 60
Context:    server config, virtual host
Status:    Core
Module:    core

For more information about the directory ,Please go through below documentation.
http://httpd.apache.org/docs/2.4/mod/mod_reqtimeout.html