In Trados Studio, you are trying to connect to Language Cloud (Trados Team/ Enterprise/ Accelerate).
NOTE: This article might also apply in certain situations when trying to connect to a GroupShare server via a proxy.

The login screen is displayed correctly, but when you try to sign in, you get an error, or when you try to add Language Cloud to a project, you get the error below. 

There was a connection protocol error. Please check that you have entered the correct port number and other settings.
(407) Proxy Authentication Required


Alternatively, in newer versions, you could be seeing different messages saying that Studio was not able to connect to Language Cloud and offering you the option to browse to the LC website.
In the Studio log file (File > View Log File) you will see an error message containing the text: (407) Proxy Authentication Required.

Go to the following location:
 C:\Program Files (x86)\SDL\SDL Trados Studio:

• \Studio4 for Trados Studio 2015
• \Studio5 for Trados Studio 2017
• \Studio15 for Trados Studio 2019
• \Studio16 for Trados Studio 2021

C:\Program Files (x86)\Trados\Trados Studio

• \Studio17 for Trados Studio 2022
• \Studio18 for Trados Studio 2024

Find SDLTradosStudio.exe.config and open it in a text editor.

Add the <defaultProxy> tag in the <system.net> element:

<defaultProxy useDefaultCredentials="true" >
</defaultProxy>

After that save and close the file.

The <system.net> element should look like this:

<system.net>
    <settings>
      <servicePointManager expect100Continue="false" />
    </settings>
    <defaultProxy useDefaultCredentials="true" >
    </defaultProxy>
</system.net>

NOTE: You will require administrator rights on the machine where this change is performed.  

When trying to log in, Language Cloud asks for authentication via the proxy, but Trados Studio itself doesn't have any options for authentication.
So the issue is resolved by telling Trados Studio to use the Default Credentials for the proxy.