Summary
When loading the FlexNet Manager Suite WebUi an "Internet Explorer cannot display the webpage" message is displayed.Symptoms
When loading the FlexNet Manager Suite WebUi an "Internet Explorer cannot display the webpage" message is displayed.
In addition to this error message when reviewing the IIS logs a 500.21 error message is displayed along with a 403 error, an example of the message is below:
"#Fields: date time s-ip cs-method cs-uri-stem cs-uri-query s-port cs-username c-ip cs(User-Agent) sc-status sc-substatus sc-win32-status time-taken
2015-11-30 00:00:01 192.168.40.114 OPTIONS /ManageSoftRL/ActiveDirectory - 80 - 192.168.40.133 FlexNet+Manager+Platform/11.0+(Windows+x86) 500 21 64 1
2015-11-30 00:00:01 192.168.40.114 OPTIONS /ManageSoftRL - 80 - 192.168.40.133 FlexNet+Manager+Platform/11.0+(Windows+x86) 500 21 64 1
2015-11-30 00:00:01 192.168.40.114 MKCOL /ManageSoftRL - 80 - 192.168.40.133 FlexNet+Manager+Platform/11.0+(Windows+x86) 403 2 64 1"
Cause
This error message is caused by ASP.Net not being registered correctly with IIS and not appearing in the Application Pools, the screenshot below outlines an expected configuration, in this example ASP.Net 4.0 is used and correctly registered and appears in the Application Pools. 
Resolution
To resolve the issue ensure the version of ASP.Net used on the machine where FlexNet Manager has been installed on is correctly registered with IIS.
The following command can be used to register .NET 4.0, if a different version of .NET is installed on the machine please consult the FNMS Install Guide in the "Configure IIS" section.
1: Open an Administrator command prompt and browse to the following directory "%SystemRoot%\Microsoft.NET\Framework\v4.0.30319", the 32 bit version needs to be used.
2: Run the following command "aspnet_regiis.exe -ir -enable".
3: Restarted IIS using the following command: "iisreset".
Workaround
The following command can be used to verify what version of .NET have been registered and the status of all versions of ASP.NET installed on the computer. The following screenshot shows all the correct versions installed if .NET 4.0 is used.
Command:
aspnet_regiis.exe -lv
Installed Versions:
