This blog post describes one of the common mistakes that you can face when not setting up IIS properly on your hosting environment.
Recently I had to investigate a deployment problem of a Sitefinity project on Windows Server 2012 R2 that uses IIS 8.5. The front-end was running successfully but when I authenticated and went to the Pages section in the back-end, I faced the following pop-up:
IIS 8.5 Detailed error - 404.0 - Not Found
The problem is caused by missing registration of managed handler in IIS.
To prevent this, you need to follow carefully the Sitefinity documentation for setting up IIS properly when deploying projects.
If you also face the same problem, see Step 13 from the doc article mentioned above or Rob Gregory's blog post for solution.