There are two solutions to this which generally work for us: the first one is regarding permissions and the second is when an incorrect . NET Framework has been set.
- Open IIS
- Add your website
- Click on Edit Permissions (top right)
Click on Security
Click Find Now
Look for IIS_IUSRS
Click OK again
Give IIS_IUSRS full rights
Click Application Pools (top left)
Click Your Website
Click Basic Settings (right column)
Set the correct .NetFrameWork for your application.
If these have solved the problem, you're done!