I am in a bizzare situation. Here's my scenario:

I have been working on an MVC5 Web API2 Project hosted on Rackspace hosting. According to the diagram I've added, the API project is AppOne. We have two data sources, one is MongoDB hosted in a Linux Server using MongoDB Driver 1.10.0.62. Another data source is a MySQL DB of the same Rackspace hosting. Now, I have a client app which is ClientApp in my diagram. From ClientAPp, I push and pull data using AppOne's APIs. It works perfectly for testing purpose. At this stage I have

The above setup works perfectly for me. The company I've been working for decided to transfer AppOne(API Project) to their own windows server. So from now on, I have a Windows Server 2012 R2 with IIS8.5 to host the AppOne(API Project). FYI, the windows server is installed with 16gigs RAM & SDDs as storage. So at this stage I have

Problem: At this stage, if I call the APIs from one machine, it works as expected. The APIs supposed to return JSON data and it returns them well. But If I call the APIs more than one PC at the same time, it returns data to only one machine and all others get http 500 error without any error details.

Please note that, I've changed the request limit to allow infinite in the IIS8.5 management console. But it still the same problem.

I hope I could describe my problem and please guys if anyone has any idea let me know.