In v9 of Enterprise Manager, you might get an authentication prompt similar to the one below (the look of the prompt depends on your internet browser).
To force the use of HTML form-based authentication, use the solution below.

The window is shown due to the automatic NTLM-based authentication method introduced in v9.

For 9.5 update 4 and later form-based authentication can be disabled solely by changing value of the following parameter in web.config file ( default location C:\Program Files\Veeam\Backup and Replication\Enterprise Manager\WebApp) to false:
<add key="useWindowsAuth" value="true"></add>

For earlier versions:

1. Open the Internet Information Services (IIS) Manager console on your Enterprise Manager server.
2. Locate the VeeamBackup site.
3. In the Default Document options, add Login.aspx to the top of the list.
4. In the Authentication options, disable Forms Authentication method.
5. In the Modules, remove the MixedAuthenticationModule2 module.
6. Open the following files using notepad and replace "<a href="/">Home</a>" with "<a href="/default.aspx">Home</a>" :
   %programfiles%\Veeam\Backup and Replication\Enterprise Manager\WebApp\js\veeam-configuration.js 
   %programfiles%\Veeam\Backup and Replication\Enterprise Manager\WebApp\js\release\app.min.js 
7. Restart the VeeamBackup website using IIS and clear the cache of your Internet browser.

Alternatively, you can go directly to the page at https://enterprise-manager:9443/login.aspx.

NTLM-based method automatically logs in the current user if the following conditions are met:
1) The computer you are using to access Enterprise Manager and EM itself are in the same AD domain
2) The user has permissions to access Enterprise Manager