IIS Server 500 Internal Server Error Access Denied

I tried this and this and this and this and countless other searched terms for getting a 500 Internal Server error. Using F12 developer tools on the browser and when the dropdown need to fetch the next dropdown's data (i.e the GET) then I get this. $ GET http://webserver/app/Home/GetConstituenciesList?Region=13 500 (Internal Server Error) This code is also present: 0x80070005

Detail:

I have a IIS web server, 2016, worked 100%, no issues, no problems, my project worked 100%. Then, I added a extra field and published and it suddenly gives me these errors.

• I deleted the 'application' from iis, re-added with a different name
• restarted iis
• restarted and cleared out the app pools
• restarted the whole server
• investigated the applicationHost file - nothing in there
• checked the link via someone else's PC signed in with a different username, on same domain - nothing
• I uninstalled the whole IIS webserver and Re-installed it again - nothing
• also added cors in the header and it literally does nothing
• VISITED ALL STACKOVERFLOW SUGGESTIONS ON THIS ISSUE, APPLIED THEM AND THEY HAVE NOT WORKED.