Error has occurred when opening findIT S2 Admin Tool

Error has occured - Client found response content type of 'text/html; charset=utf-8', but expected 'text/xml'

or  500.19 

HTTP Error 500.19  - Internal Server Error

Such as this:

When opening the admin tool and clicking login

This is is an error caused by the .net version incompatibility, check if your .net version is set to 4.0, findIT S2 was written with .net 3.5 so requires 2.0 compatability which works with up to 3.5

Resolution:

1. Launch IIS manager
2. click on Application Pools
3. Right click on the findIT S2 Application Pool and choose advanced settings
   
4. change the .net version to 2.0, then click ok
5. click to recycle the app pool then try re opening the admin tool