>Just to note that the title is ambiguous. Until I've read the message, I couldn't decide whether the title means
>
>- which thing exactly is failing the f5 check?
>- what does "failing f5 check" mean, really?

The bottom line I still don't understand the concept of the "F5 health check" This is how the customer IT refers to it. He tried to explain it to me a few days ago but it mostly went over my head. The important thing is that he fixed the problem with my ASP.NET application deployment and now it works (keep my fingers crossed).
I installed 10 web sites on their web server; each site is completely independent and each has it's own SQL Server database. What they wanted is the for all users to see the URL without the port number. Here is how, after the installation, my site/application URL usually looks like (9090 is a sample port number):

http://servername:9090/MainPage.aspx

They (IT) wanted the URL to be like this:

https://applicationname3letterid.organziation.org

They installed SSL so that the http becomes https. the 3-letter id distinguishes each of the 10 sites. So that each URL (for the 10 sites) has a different 3-letter id. This id is not arbitrary; they have a system.
And as you can see from their URL (above), the main page name is not even part of the URL. As long as it works for them, I am happy :)