Compare the NTFS security permissions for the folder on the old and new servers.
Your image shows "HTTP ERROR 500", so check the http error logs in C:\Windows\System32\LogFiles\HTTPERR for additional error information.
Also check the IIS logs and the Application and System event logs for errors.
There should be sub codes and an error description.
https://www.bing.com/search?pglt=41&q=how+to+troubleshoot+iis+500+error
Configure IIS to send detailed errors back to the browser.