Once you have upgraded from one version of Windows Server to a newer version (say, for example, Windows 2008 R2 to Windows 2012), and installed BizTalk360 on the new set up, you may see an error message as follows -
and when trying to reach http://localhost/BizTalk360/Services.REST/AdminService.svc, if you see the following error
Download and install the .Net Framework 4.5 version (and above) again this should solve your issue. After upgrading the Windows server, .Net Framework might not be updated along with the Windows server upgrade. Therefore, a fresh installation of .Net Framework will help to resolve this issue.